PL/pgSQL is a loadable procedural language for the Postgres database system. This package was originally written by Jan Wieck. This documentation was in part written by Roberto Mello (rmello@fslc.usu.edu). The design goals of PL/pgSQL were to create a loadable procedural language that can be used to create functions and trigger procedures, adds control structures to the SQL language, can perform complex computations, inherits all user defined types, functions and operators, can be defined to be trusted by the server, is easy to use. The PL/pgSQL call handler parses the function's source text and produces an internal binary instruction tree the first time the function is called. The produced bytecode is identified in the call handler by the object ID of the function. This ensures that changing a function by a DROP/CREATE sequence will take effect without establishing a new database connection. For all expressions and SQL statements used in the function, the PL/pgSQL bytecode interpreter creates a prepared execution plan using the SPI manager's SPI_prepare() and SPI_saveplan() functions. This is done the first time the individual statement is processed in the PL/pgSQL function. Thus, a function with conditional code that contains many statements for which execution plans would be required, will only prepare and save those plans that are really used during the lifetime of the database connection. This means that you have to be careful about your user-defined functions. For example: CREATE FUNCTION populate() RETURNS INTEGER AS ' DECLARE -- Declarations BEGIN PERFORM my_function(); END; ' LANGUAGE 'plpgsql'; If you create the above function, it will reference the OID for my_function() in its bytecode. Later, if you drop and re-create my_function(), then populate() will not be able to find my_function() anymore. You would then have to re-create populate(). Because PL/pgSQL saves execution plans in this way, queries that appear directly in a PL/pgSQL function must refer to the same tables and fields on every execution; that is, you cannot use a parameter as the name of a table or field in a query. To get around this restriction, you can construct dynamic queries using the PL/pgSQL EXECUTE statement --- at the price of constructing a new query plan on every execution. Except for input/output conversion and calculation functions for user defined types, anything that can be defined in C language functions can also be done with PL/pgSQL. It is possible to create complex conditional computation functions and later use them to define operators or use them in functional indices. Better performance (see ) SQL support (see ) Portability (see ) SQL is the language PostgreSQL (and most other Relational Databases) use as query language. It's portable and easy to learn. But every SQL statement must be executed individually by the database server. That means that your client application must send each query to the database server, wait for it to process it, receive the results, do some computation, then send other queries to the server. All this incurs inter process communication and may also incur network overhead if your client is on a different machine than the database server. With PL/pgSQL you can group a block of computation and a series of queries inside the database server, thus having the power of a procedural language and the ease of use of SQL, but saving lots of time because you don't have the whole client/server communication overhead. Your application will enjoy a considerable performance increase by using PL/pgSQL. PL/pgSQL adds the power of a procedural language to the flexibility and ease of SQL. With PL/pgSQL you can use all the datatypes, columns, operators and functions of SQL. Because PL/pgSQL functions run inside PostgreSQL, these functions will run on any platform where PostgreSQL runs. Thus you can reuse code and have less development costs. Developing in PL/pgSQL is pretty straight forward, especially if you have developed in other database procedural languages, such as Oracle's PL/SQL. Two good ways of developing in PL/pgSQL are: Using a text editor and reloading the file with psql Using PostgreSQL's GUI Tool: pgaccess One good way to develop in PL/pgSQL is to simply use the text editor of your choice to create your functions, and in another console, use psql (PostgreSQL's interactive monitor) to load those functions. If you are doing it this way (and if you are a PL/pgSQL novice or in debugging stage), it is a good idea to always DROP your function before creating it. That way when you reload the file, it'll drop your functions and then re-create them. For example: drop function testfunc(integer); create function testfunc(integer) return integer as ' .... end; ' language 'plpgsql'; When you load the file for the first time, PostgreSQL will raise a warning saying this function doesn't exist and go on to create it. To load an SQL file (filename.sql) into a database named "dbname", use the command: psql -f filename.sql dbname Another good way to develop in PL/pgSQL is using PostgreSQL's GUI tool: pgaccess. It does some nice things for you, like escaping single-quotes, and making it easy to recreate and debug functions. PL/pgSQL is a block structured language. All keywords and identifiers can be used in mixed upper and lower-case. A block is defined as: <<label>> DECLARE declarations BEGIN statements END; There can be any number of sub-blocks in the statement section of a block. Sub-blocks can be used to hide variables from outside a block of statements. The variables declared in the declarations section preceding a block are initialized to their default values every time the block is entered, not only once per function call. For example: CREATE FUNCTION somefunc() RETURNS INTEGER AS ' DECLARE quantity INTEGER := 30; BEGIN RAISE NOTICE ''Quantity here is %'',quantity; -- Quantity here is 30 quantity := 50; -- -- Create a sub-block -- DECLARE quantity INTEGER := 80; BEGIN RAISE NOTICE ''Quantity here is %'',quantity; -- Quantity here is 80 END; RAISE NOTICE ''Quantity here is %'',quantity; -- Quantity here is 50 END; ' LANGUAGE 'plpgsql'; It is important not to confuse the use of BEGIN/END for grouping statements in PL/pgSQL with the database commands for transaction control. PL/pgSQL's BEGIN/END are only for grouping; they do not start or end a transaction. Functions and trigger procedures are always executed within a transaction established by an outer query --- they cannot start or commit transactions, since Postgres does not have nested transactions. There are two types of comments in PL/pgSQL. A double dash -- starts a comment that extends to the end of the line. A /* starts a block comment that extends to the next occurrence of */. Block comments cannot be nested, but double dash comments can be enclosed into a block comment and a double dash can hide the block comment delimiters /* and */. All variables, rows and records used in a block or its sub-blocks must be declared in the declarations section of a block. The exception being the loop variable of a FOR loop iterating over a range of integer values. PL/pgSQL variables can have any SQL datatype, such as INTEGER, VARCHAR and CHAR. All variables have as default value the SQL NULL value. Here are some examples of variable declarations: user_id INTEGER; quantity NUMBER(5); url VARCHAR; The declarations have the following syntax: name CONSTANT type NOT NULL { DEFAULT | := } value ; The value of variables declared as CONSTANT cannot be changed. If NOT NULL is specified, an assignment of a NULL value results in a runtime error. Since the default value of all variables is the SQL NULL value, all variables declared as NOT NULL must also have a default value specified. The default value is evaluated every time the function is called. So assigning 'now' to a variable of type timestamp causes the variable to have the time of the actual function call, not when the function was precompiled into its bytecode. Examples: quantity INTEGER := 32; url varchar := ''http://mysite.com''; user_id CONSTANT INTEGER := 10; Variables passed to functions are named with the identifiers $1, $2, etc. (maximum is 16). Some examples: CREATE FUNCTION sales_tax(REAL) RETURNS REAL AS ' DECLARE subtotal ALIAS FOR $1; BEGIN return subtotal * 0.06; END; ' LANGUAGE 'plpgsql'; CREATE FUNCTION instr(VARCHAR,INTEGER) RETURNS INTEGER AS ' DECLARE v_string ALIAS FOR $1; index ALIAS FOR $2; BEGIN -- Some computations here END; ' LANGUAGE 'plpgsql'; Using the %TYPE and %ROWTYPE attributes, you can declare variables with the same datatype or structure of another database item (e.g: a table field). %TYPE %TYPE provides the datatype of a variable or database column. You can use this to declare variables that will hold database values. For example, let's say you have a column named user_id in your users table. To declare a variable with the same datatype as users you do: user_id users.user_id%TYPE; By using %TYPE you don't need to know the datatype of the structure you are referencing, and most important, if the datatype of the referenced item changes in the future (e.g: you change your table definition of user_id to become a REAL), you won't need to change your function definition. name table%ROWTYPE; Declares a row with the structure of the given table. table must be an existing table or view name of the database. The fields of the row are accessed in the dot notation. Parameters to a function can be composite types (complete table rows). In that case, the corresponding identifier $n will be a rowtype, but it must be aliased using the ALIAS command described above. Only the user attributes of a table row are accessible in the row, no OID or other system attributes (because the row could be from a view). The fields of the rowtype inherit the table's field sizes or precision for char() etc. data types. DECLARE users_rec users%ROWTYPE; user_id users%TYPE; BEGIN user_id := users_rec.user_id; ... create function cs_refresh_one_mv(integer) returns integer as ' DECLARE key ALIAS FOR $1; table_data cs_materialized_views%ROWTYPE; BEGIN SELECT INTO table_data * FROM cs_materialized_views WHERE sort_key=key; IF NOT FOUND THEN RAISE EXCEPTION ''View '' || key || '' not found''; RETURN 0; END IF; -- The mv_name column of cs_materialized_views stores view -- names. TRUNCATE TABLE table_data.mv_name; INSERT INTO table_data.mv_name || '' '' || table_data.mv_query; return 1; end; ' LANGUAGE 'plpgsql'; Using RENAME you can change the name of a variable, record or row. This is useful if NEW or OLD should be referenced by another name inside a trigger procedure. Syntax and examples: RENAME oldname TO newname; RENAME id TO user_id; RENAME this_var TO that_var; All expressions used in PL/pgSQL statements are processed using the backend's executor. Expressions that appear to contain constants may in fact require run-time evaluation (e.g. 'now' for the timestamp type) so it is impossible for the PL/pgSQL parser to identify real constant values other than the NULL keyword. All expressions are evaluated internally by executing a query SELECT expression using the SPI manager. In the expression, occurrences of variable identifiers are substituted by parameters and the actual values from the variables are passed to the executor in the parameter array. All expressions used in a PL/pgSQL function are only prepared and saved once. The only exception to this rule is an EXECUTE statement if parsing of a query is needed each time it is encountered. The type checking done by the Postgres main parser has some side effects to the interpretation of constant values. In detail there is a difference between what these two functions do: CREATE FUNCTION logfunc1 (text) RETURNS timestamp AS ' DECLARE logtxt ALIAS FOR $1; BEGIN INSERT INTO logtable VALUES (logtxt, ''now''); RETURN ''now''; END; ' LANGUAGE 'plpgsql'; and CREATE FUNCTION logfunc2 (text) RETURNS timestamp AS ' DECLARE logtxt ALIAS FOR $1; curtime timestamp; BEGIN curtime := ''now''; INSERT INTO logtable VALUES (logtxt, curtime); RETURN curtime; END; ' LANGUAGE 'plpgsql'; In the case of logfunc1(), the Postgres main parser knows when preparing the plan for the INSERT, that the string 'now' should be interpreted as timestamp because the target field of logtable is of that type. Thus, it will make a constant from it at this time and this constant value is then used in all invocations of logfunc1() during the lifetime of the backend. Needless to say that this isn't what the programmer wanted. In the case of logfunc2(), the Postgres main parser does not know what type 'now' should become and therefore it returns a data type of text containing the string 'now'. During the assignment to the local variable curtime, the PL/pgSQL interpreter casts this string to the timestamp type by calling the text_out() and timestamp_in() functions for the conversion. This type checking done by the Postgres main parser got implemented after PL/pgSQL was nearly done. It is a difference between 6.3 and 6.4 and affects all functions using the prepared plan feature of the SPI manager. Using a local variable in the above manner is currently the only way in PL/pgSQL to get those values interpreted correctly. If record fields are used in expressions or statements, the data types of fields should not change between calls of one and the same expression. Keep this in mind when writing trigger procedures that handle events for more than one table. Anything not understood by the PL/pgSQL parser as specified below will be put into a query and sent down to the database engine to execute. The resulting query should not return any data. An assignment of a value to a variable or row/record field is written as: identifier := expression; If the expressions result data type doesn't match the variables data type, or the variable has a size/precision that is known (as for char(20)), the result value will be implicitly casted by the PL/pgSQL bytecode interpreter using the result types output- and the variables type input-functions. Note that this could potentially result in runtime errors generated by the types input functions. user_id := 20; tax := subtotal * 0.06; All functions defined in a Postgres database return a value. Thus, the normal way to call a function is to execute a SELECT query or doing an assignment (resulting in a PL/pgSQL internal SELECT). But there are cases where someone is not interested in the function's result. In these cases, use the PERFORM statement. PERFORM query This executes a SELECT query over the SPI manager and discards the result. Identifiers like local variables are still substituted into parameters. PERFORM create_mv(''cs_session_page_requests_mv'','' select session_id, page_id, count(*) as n_hits, sum(dwell_time) as dwell_time, count(dwell_time) as dwell_count from cs_fact_table group by session_id, page_id ''); Often times you will want to generate dynamic queries inside your PL/pgSQL functions. Or you have functions that will generate other functions. PL/pgSQL provides the EXECUTE statement for these occasions. EXECUTE query-string where query-string is a string of type text containing the query to be executed. When working with dynamic queries you will have to face escaping of single quotes in PL/pgSQL. Please refer to the table available at the "Porting from Oracle PL/SQL" chapter for a detailed explanation that will save you some effort. Unlike all other queries in PL/pgSQL, a query run by an EXECUTE statement is not prepared and saved just once during the life of the server. Instead, the query is prepared each time the statement is run. The query-string can be dynamically created within the procedure to perform actions on variable tables and fields. The results from SELECT queries are discarded by EXECUTE, and SELECT INTO is not currently supported within EXECUTE. So, the only way to extract a result from a dynamically-created SELECT is to use the FOR ... EXECUTE form described later. An example: EXECUTE ''UPDATE tbl SET '' || quote_ident(fieldname) || '' = '' || quote_literal(newvalue) || '' WHERE ...''; This example shows use of the functions quote_ident(TEXT) and quote_literal(TEXT). Variables containing field and table identifiers should be passed to function quote_ident(). Variables containing literal elements of the dynamic query string should be passed to quote_literal(). Both take the appropriate steps to return the input text enclosed in single or double quotes and with any embedded special characters. Here is a much larger example of a dynamic query and EXECUTE: CREATE FUNCTION cs_update_referrer_type_proc() RETURNS INTEGER AS ' DECLARE referrer_keys RECORD; -- Declare a generic record to be used in a FOR a_output varchar(4000); BEGIN a_output := ''CREATE FUNCTION cs_find_referrer_type(varchar,varchar,varchar) RETURNS varchar AS '''' DECLARE v_host ALIAS FOR $1; v_domain ALIAS FOR $2; v_url ALIAS FOR $3; ''; -- -- Notice how we scan through the results of a query in a FOR loop -- using the FOR <record> construct. -- FOR referrer_keys IN select * from cs_referrer_keys order by try_order LOOP a_output := a_output || '' if v_'' || referrer_keys.kind || '' like '''''''''' || referrer_keys.key_string || '''''''''' then return '''''' || referrer_keys.referrer_type || ''''''; end if;''; END LOOP; a_output := a_output || '' return null; end; '''' language ''''plpgsql'''';''; -- This works because we are not substituting any variables -- Otherwise it would fail. Look at PERFORM for another way to run functions EXECUTE a_output; end; ' LANGUAGE 'plpgsql'; GET DIAGNOSTICS variable = item , ... This command allows retrieval of system status indicators. Each item is a keyword identifying a state value to be assigned to the specified variable (which should be of the right datatype to receive it). The currently available status items are ROW_COUNT, the number of rows processed by the last SQL query sent down to the SQL engine; and RESULT_OID, the Oid of the last row inserted by the most recent SQL query. Note that RESULT_OID is only useful after an INSERT query. RETURN expression The function terminates and the value of expression will be returned to the upper executor. The return value of a function cannot be undefined. If control reaches the end of the top-level block of the function without hitting a RETURN statement, a runtime error will occur. The expressions result will be automatically casted into the function's return type as described for assignments. Control structures are probably the most useful (and important) part of PL/SQL. With PL/pgSQL's control structures, you can manipulate PostgreSQL data in a very flexible and powerful way. IF statements let you take action according to certain conditions. PL/pgSQL has three forms of IF: IF-THEN, IF-THEN-ELSE, IF-THEN-ELSE IF. NOTE: All PL/pgSQL IF statements need a corresponding END IF statement. In ELSE-IF statements you need two: one for the first IF and one for the second (ELSE IF). IF-THEN IF-THEN statements is the simplest form of an IF. The statements between THEN and END IF will be executed if the condition is true. Otherwise, the statements following END IF will be executed. IF v_user_id <> 0 THEN UPDATE users SET email = v_email WHERE user_id = v_user_id; END IF; IF-THEN-ELSE IF-THEN-ELSE statements adds to IF-THEN by letting you specify the statements that should be executed if the condition evaluates to FALSE. IF parentid IS NULL or parentid = '''' THEN return fullname; ELSE return hp_true_filename(parentid) || ''/'' || fullname; END IF; IF v_count > 0 THEN INSERT INTO users_count(count) VALUES(v_count); return ''t''; ELSE return ''f''; END IF; IF statements can be nested and in the following example: IF demo_row.sex = ''m'' THEN pretty_sex := ''man''; ELSE IF demo_row.sex = ''f'' THEN pretty_sex := ''woman''; END IF; END IF; IF-THEN-ELSE IF When you use the "ELSE IF" statement, you are actually nesting an IF statement inside the ELSE statement. Thus you need one END IF statement for each nested IF and one for the parent IF-ELSE. For example: IF demo_row.sex = ''m'' THEN pretty_sex := ''man''; ELSE IF demo_row.sex = ''f'' THEN pretty_sex := ''woman''; END IF; END IF; With the LOOP, WHILE, FOR and EXIT statements, you can control the flow of execution of your PL/pgSQL program iteratively. LOOP <<label>> LOOP statements END LOOP; An unconditional loop that must be terminated explicitly by an EXIT statement. The optional label can be used by EXIT statements of nested loops to specify which level of nesting should be terminated. EXIT EXIT label WHEN expression ; If no label is given, the innermost loop is terminated and the statement following END LOOP is executed next. If label is given, it must be the label of the current or an upper level of nested loop blocks. Then the named loop or block is terminated and control continues with the statement after the loops/blocks corresponding END. Examples: LOOP -- some computations IF count > 0 THEN EXIT; -- exit loop END IF; END LOOP; LOOP -- some computations EXIT WHEN count > 0; END LOOP; BEGIN -- some computations IF stocks > 100000 THEN EXIT; -- illegal. Can't use EXIT outside of a LOOP END IF; END; WHILE With the WHILE statement, you can loop through a sequence of statements as long as the evaluation of the condition expression is true. <<label>> WHILE expression LOOP statements END LOOP; For example: WHILE amount_owed > 0 AND gift_certificate_balance > 0 LOOP -- some computations here END LOOP; WHILE NOT boolean_expression LOOP -- some computations here END LOOP; FOR <<label>> FOR name IN REVERSE expression .. expression LOOP statements END LOOP; A loop that iterates over a range of integer values. The variable name is automatically created as type integer and exists only inside the loop. The two expressions giving the lower and upper bound of the range are evaluated only when entering the loop. The iteration step is always 1. Some examples of FOR loops (see for iterating over records in FOR loops): FOR i IN 1..10 LOOP -- some expressions here RAISE NOTICE 'i is %',i; END LOOP; FOR i IN REVERSE 1..10 LOOP -- some expressions here END LOOP; Records are similar to rowtypes, but they have no predefined structure. They are used in selections and FOR loops to hold one actual database row from a SELECT operation. One variables of type RECORD can be used for different selections. Accessing a record or an attempt to assign a value to a record field when there is no actual row in it results in a runtime error. They can be declared like this: name RECORD; An assignment of a complete selection into a record or row can be done by: SELECT INTO target expressions FROM ...; target can be a record, a row variable or a comma separated list of variables and record-/row-fields. Note that this is quite different from Postgres' normal interpretation of SELECT INTO, which is that the INTO target is a newly created table. (If you want to create a table from a SELECT result inside a PL/pgSQL function, use the equivalent syntax CREATE TABLE AS SELECT.) If a row or a variable list is used as target, the selected values must exactly match the structure of the target(s) or a runtime error occurs. The FROM keyword can be followed by any valid qualification, grouping, sorting etc. that can be given for a SELECT statement. Once a record or row has been assigned to a RECORD variable, you can use the "." (dot) notation to access fields in that record: DECLARE users_rec RECORD; full_name varchar; BEGIN SELECT INTO users_rec * FROM users WHERE user_id=3; full_name := users_rec.first_name || '' '' || users_rec.last_name; There is a special variable named FOUND of type boolean that can be used immediately after a SELECT INTO to check if an assignment had success. SELECT INTO myrec * FROM EMP WHERE empname = myname; IF NOT FOUND THEN RAISE EXCEPTION ''employee % not found'', myname; END IF; You can also use the IS NULL (or ISNULL) conditionals to test for NULLity of a RECORD/ROW. If the selection returns multiple rows, only the first is moved into the target fields. All others are silently discarded. DECLARE users_rec RECORD; full_name varchar; BEGIN SELECT INTO users_rec * FROM users WHERE user_id=3; IF users_rec.homepage IS NULL THEN -- user entered no homepage, return "http://" return ''http://''; END IF; END; Using a special type of FOR loop, you can iterate through the results of a query and manipulate that data accordingly. The syntax is as follow: <<label>> FOR record | row IN select_clause LOOP statements END LOOP; The record or row is assigned all the rows resulting from the select clause and the loop body executed for each. Here is an example: create function cs_refresh_mviews () returns integer as ' DECLARE mviews RECORD; -- Instead, if you did: -- mviews cs_materialized_views%ROWTYPE; -- this record would ONLY be usable for the cs_materialized_views table BEGIN PERFORM cs_log(''Refreshing materialized views...''); FOR mviews IN SELECT * FROM cs_materialized_views ORDER BY sort_key LOOP -- Now "mviews" has one record from cs_materialized_views PERFORM cs_log(''Refreshing materialized view '' || mview.mv_name || ''...''); TRUNCATE TABLE mview.mv_name; INSERT INTO mview.mv_name || '' '' || mview.mv_query; END LOOP; PERFORM cs_log(''Done refreshing materialized views.''); return 1; end; ' language 'plpgsql'; If the loop is terminated with an EXIT statement, the last assigned row is still accessible after the loop. The FOR-IN EXECUTE statement is another way to iterate over records: <<label>> FOR record | row IN EXECUTE text_expression LOOP statements END LOOP; This is like the previous form, except that the source SELECT statement is specified as a string expression, which is evaluated and re-planned on each entry to the FOR loop. This allows the programmer to choose the speed of a pre-planned query or the flexibility of a dynamic query, just as with a plain EXECUTE statement. Use the RAISE statement to throw messages into the Postgres elog mechanism. RAISE level 'format' , identifier ...; Inside the format, % is used as a placeholder for the subsequent comma-separated identifiers. Possible levels are DEBUG (silently suppressed in production running databases), NOTICE (written into the database log and forwarded to the client application) and EXCEPTION (written into the database log and aborting the transaction). RAISE NOTICE ''Id number '' || key || '' not found!''; RAISE NOTICE ''Calling cs_create_job(%)'',v_job_id; In this last example, v_job_id will replace the % in the string. RAISE EXCEPTION ''Inexistent ID --> %'',user_id; This will abort the transaction and write to the database log. Postgres does not have a very smart exception handling model. Whenever the parser, planner/optimizer or executor decide that a statement cannot be processed any longer, the whole transaction gets aborted and the system jumps back into the main loop to get the next query from the client application. It is possible to hook into the error mechanism to notice that this happens. But currently it is impossible to tell what really caused the abort (input/output conversion error, floating point error, parse error). And it is possible that the database backend is in an inconsistent state at this point so returning to the upper executor or issuing more commands might corrupt the whole database. And even if, at this point the information, that the transaction is aborted, is already sent to the client application, so resuming operation does not make any sense. Thus, the only thing PL/pgSQL currently does when it encounters an abort during execution of a function or trigger procedure is to write some additional DEBUG level log messages telling in which function and where (line number and type of statement) this happened. PL/pgSQL can be used to define trigger procedures. They are created with the usual CREATE FUNCTION command as a function with no arguments and a return type of OPAQUE. There are some Postgres specific details in functions used as trigger procedures. First they have some special variables created automatically in the top-level blocks declaration section. They are NEW Data type RECORD; variable holding the new database row on INSERT/UPDATE operations on ROW level triggers. OLD Data type RECORD; variable holding the old database row on UPDATE/DELETE operations on ROW level triggers. TG_NAME Data type name; variable that contains the name of the trigger actually fired. TG_WHEN Data type text; a string of either BEFORE or AFTER depending on the triggers definition. TG_LEVEL Data type text; a string of either ROW or STATEMENT depending on the triggers definition. TG_OP Data type text; a string of INSERT, UPDATE or DELETE telling for which operation the trigger is actually fired. TG_RELID Data type oid; the object ID of the table that caused the trigger invocation. TG_RELNAME Data type name; the name of the table that caused the trigger invocation. TG_NARGS Data type integer; the number of arguments given to the trigger procedure in the CREATE TRIGGER statement. TG_ARGV[] Data type array of text; the arguments from the CREATE TRIGGER statement. The index counts from 0 and can be given as an expression. Invalid indices (< 0 or >= tg_nargs) result in a NULL value. Second they must return either NULL or a record/row containing exactly the structure of the table the trigger was fired for. Triggers fired AFTER might always return a NULL value with no effect. Triggers fired BEFORE signal the trigger manager to skip the operation for this actual row when returning NULL. Otherwise, the returned record/row replaces the inserted/updated row in the operation. It is possible to replace single values directly in NEW and return that or to build a complete new record/row to return. This trigger ensures, that any time a row is inserted or updated in the table, the current user name and time are stamped into the row. And it ensures that an employees name is given and that the salary is a positive value. CREATE TABLE emp ( empname text, salary integer, last_date timestamp, last_user text ); CREATE FUNCTION emp_stamp () RETURNS OPAQUE AS ' BEGIN -- Check that empname and salary are given IF NEW.empname ISNULL THEN RAISE EXCEPTION ''empname cannot be NULL value''; END IF; IF NEW.salary ISNULL THEN RAISE EXCEPTION ''% cannot have NULL salary'', NEW.empname; END IF; -- Who works for us when she must pay for? IF NEW.salary < 0 THEN RAISE EXCEPTION ''% cannot have a negative salary'', NEW.empname; END IF; -- Remember who changed the payroll when NEW.last_date := ''now''; NEW.last_user := current_user; RETURN NEW; END; ' LANGUAGE 'plpgsql'; CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp FOR EACH ROW EXECUTE PROCEDURE emp_stamp(); Here are only a few functions to demonstrate how easy it is to write PL/pgSQL functions. For more complex examples the programmer might look at the regression test for PL/pgSQL. One painful detail in writing functions in PL/pgSQL is the handling of single quotes. The function's source text on CREATE FUNCTION must be a literal string. Single quotes inside of literal strings must be either doubled or quoted with a backslash. We are still looking for an elegant alternative. In the meantime, doubling the single quotes as in the examples below should be used. Any solution for this in future versions of Postgres will be forward compatible. For a detailed explanation and examples of how to escape single quotes in different situations, please see . The following two PL/pgSQL functions are identical to their counterparts from the C language function discussion. This function receives an integer and increments it by one, returning the incremented value. CREATE FUNCTION add_one (integer) RETURNS integer AS ' BEGIN RETURN $1 + 1; END; ' LANGUAGE 'plpgsql'; This function receives two text parameters and returns the result of concatenating them. CREATE FUNCTION concat_text (text, text) RETURNS text AS ' BEGIN RETURN $1 || $2; END; ' LANGUAGE 'plpgsql'; In this example, we take EMP (a table) and an integer as arguments to our function, which returns a boolean. If the "salary" field of the EMP table is NULL, we return "f". Otherwise we compare with that field with the integer passed to the function and return the boolean result of the comparison (t or f). This is the PL/pgSQL equivalent to the example from the C functions. CREATE FUNCTION c_overpaid (EMP, integer) RETURNS boolean AS ' DECLARE emprec ALIAS FOR $1; sallim ALIAS FOR $2; BEGIN IF emprec.salary ISNULL THEN RETURN ''f''; END IF; RETURN emprec.salary > sallim; END; ' LANGUAGE 'plpgsql'; February 2001 Roberto Mello

rmello@fslc.usu.edu

Except for portions of this document quoted from other sources, this document is licensed under the BSD License. Roberto Mello (rmello@fslc.usu.edu) This section explains differences between Oracle's PL/SQL and PostgreSQL's PL/pgSQL languages in the hopes of helping developers port applications from Oracle to PostgreSQL. Most of the code here is from the ArsDigita Clickstream module that I ported to PostgreSQL when I took an internship with OpenForce Inc. in the Summer of 2000. PL/pgSQL is similar to PL/SQL in many aspects. It is a block structured, imperative language (all variables have to be declared). PL/SQL has many more features than its PostgreSQL counterpart, but PL/pgSQL allows for a great deal of functionality and it is being improved constantly. Some things you should keep in mind when porting from Oracle to PostgreSQL: No default parameters in PostgreSQL. You can overload functions in PostgreSQL. This is often used to work around the lack of default parameters. Assignments, loops and conditionals are similar. No need for cursors in PostgreSQL, just put the query in the FOR statement (see example below) In PostgreSQL you need to escape single quotes. See . In PostgreSQL you need to escape single quotes inside your function definition. This can lead to quite amusing code at times, especially if you are creating a function that generates other function(s), as in . One thing to keep in mind when escaping lots of single quotes is that, except for the beginning/ending quotes, all the others will come in even quantity. gives the scoop. (You'll love this little chart.) No. of Quotes Usage Example Result 1 To begin/terminate function bodies CREATE FUNCTION foo() RETURNS INTEGER AS '...' LANGUAGE 'plpgsql'; as is 2 In assignments, SELECTs, to delimit strings, etc. a_output := ''Blah''; SELECT * FROM users WHERE f_name=''foobar''; SELECT * FROM users WHERE f_name='foobar'; 4 When you need two single quotes in your resulting string without terminating that string. a_output := a_output || '' AND name LIKE ''''foobar'''' AND ...'' AND name LIKE 'foobar' AND ... 6 When you want double quotes in your resulting string and terminate that string. a_output := a_output || '' AND name LIKE ''''foobar'''''' AND name LIKE 'foobar' 10 When you want two single quotes in the resulting string (which accounts for 8 quotes) and terminate that string (2 more). You will probably only need that if you were using a function to generate other functions (like in ). a_output := a_output || '' if v_'' || referrer_keys.kind || '' like '''''''''' || referrer_keys.key_string || '''''''''' then return '''''' || referrer_keys.referrer_type || ''''''; end if;''; if v_<...> like ''<...>'' then return ''<...>''; end if; Here is an Oracle function: CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name IN varchar, v_version IN varchar) RETURN varchar IS BEGIN IF v_version IS NULL THEN RETURN v_name; END IF; RETURN v_name || '/' || v_version; END; / SHOW ERRORS; Let's go through this function and see the differences to PL/pgSQL: The OR REPLACE clause is not allowed. You will have to explicitly drop the function before creating it to achieve similar results. PostgreSQL does not have named parameters. You have to explicitly alias them inside your function. Oracle can have IN, OUT, and INOUT parameters passed to functions. The INOUT, for example, means that the parameter will receive a value and return another. PostgreSQL only has IN parameters and functions can return only a single value. The RETURN key word in the function prototype (not the function body) becomes RETURNS in PostgreSQL. On PostgreSQL functions are created using single quotes as delimiters, so you have to escape single quotes inside your functions (which can be quite annoying at times; see ). The /show errors command does not exist in PostgreSQL. So let's see how this function would be look like ported to PostgreSQL: DROP FUNCTION cs_fmt_browser_version(varchar, varchar); CREATE FUNCTION cs_fmt_browser_version(varchar, varchar) RETRUNS varchar AS ' DECLARE v_name ALIAS FOR $1; v_version ALIAS FOR $2; BEGIN IF v_version IS NULL THEN return v_name; END IF; RETURN v_name || ''/'' || v_version; END; ' LANGUAGE 'plpgsql'; The following procedure grabs rows from a SELECT statement and builds a large function with the results in IF statements, for the sake of efficiency. Notice particularly the differences in cursors, FOR loops, and the need to escape single quotes in PostgreSQL. create or replace procedure cs_update_referrer_type_proc is cursor referrer_keys is select * from cs_referrer_keys order by try_order; a_output varchar(4000); begin a_output := 'create or replace function cs_find_referrer_type(v_host IN varchar, v_domain IN varchar, v_url IN varchar) return varchar is begin'; for referrer_key in referrer_keys loop a_output := a_output || ' if v_' || referrer_key.kind || ' like ''' || referrer_key.key_string || ''' then return ''' || referrer_key.referrer_type || '''; end if;'; end loop; a_output := a_output || ' return null; end;'; execute immediate a_output; end; / show errors Here is how this function would end up in PostgreSQL: CREATE FUNCTION cs_update_referrer_type_proc() RETURNS integer AS ' DECLARE referrer_keys RECORD; -- Declare a generic record to be used in a FOR a_output varchar(4000); BEGIN a_output := ''CREATE FUNCTION cs_find_referrer_type(varchar,varchar,varchar) RETURNS varchar AS '''' DECLARE v_host ALIAS FOR $1; v_domain ALIAS FOR $2; v_url ALIAS FOR $3; ''; -- -- Notice how we scan through the results of a query in a FOR loop -- using the FOR <record> construct. -- FOR referrer_keys IN select * from cs_referrer_keys order by try_order LOOP a_output := a_output || '' if v_'' || referrer_keys.kind || '' like '''''''''' || referrer_keys.key_string || '''''''''' then return '''''' || referrer_keys.referrer_type || ''''''; end if;''; END LOOP; a_output := a_output || '' return null; end; '''' language ''''plpgsql'''';''; -- This works because we are not substituting any variables -- Otherwise it would fail. Look at PERFORM for another way to run functions EXECUTE a_output; end; ' LANGUAGE 'plpgsql'; The following Oracle PL/SQL procedure is used to parse a URL and return several elements (host, path and query). It is an procedure because in PL/pgSQL functions only one value can be returned (see ). In PostgreSQL, one way to work around this is to split the procedure in three different functions: one to return the host, another for the path and another for the query. create or replace procedure cs_parse_url( v_url IN varchar, v_host OUT varchar, -- This will be passed back v_path OUT varchar, -- This one too v_query OUT varchar) -- And this one is a_pos1 integer; a_pos2 integer; begin v_host := NULL; v_path := NULL; v_query := NULL; a_pos1 := instr(v_url, '//'); -- PostgreSQL doesn't have an instr function if a_pos1 = 0 then return; end if; a_pos2 := instr(v_url, '/', a_pos1 + 2); if a_pos2 = 0 then v_host := substr(v_url, a_pos1 + 2); v_path := '/'; return; end if; v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2); a_pos1 := instr(v_url, '?', a_pos2 + 1); if a_pos1 = 0 then v_path := substr(v_url, a_pos2); return; end if; v_path := substr(v_url, a_pos2, a_pos1 - a_pos2); v_query := substr(v_url, a_pos1 + 1); end; / show errors; Here is how this procedure could be translated for PostgreSQL: drop function cs_parse_url_host(varchar); create function cs_parse_url_host(varchar) returns varchar as ' declare v_url ALIAS FOR $1; v_host varchar; v_path varchar; a_pos1 integer; a_pos2 integer; a_pos3 integer; begin v_host := NULL; a_pos1 := instr(v_url,''//''); if a_pos1 = 0 then return ''''; -- Return a blank end if; a_pos2 := instr(v_url,''/'',a_pos1 + 2); if a_pos2 = 0 then v_host := substr(v_url, a_pos1 + 2); v_path := ''/''; return v_host; end if; v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2 ); return v_host; end; ' language 'plpgsql'; PostgreSQL does not have an instr function, so you can work around it using a combination of other functions. I got tired of doing this and created my own instr functions that behave exactly like Oracle's (it makes life easier). See the for the code. Oracle procedures give a little more flexibility to the developer because nothing needs to be explicitly returned, but it can be through the use of INOUT or OUT parameters. An example: create or replace procedure cs_create_job(v_job_id in integer) is a_running_job_count integer; pragma autonomous_transaction; begin lock table cs_jobs in exclusive mode; select count(*) into a_running_job_count from cs_jobs where end_stamp is null; if a_running_job_count > 0 then commit; -- free lock raise_application_error(-20000, 'Unable to create a new job: a job is currently running.'); end if; delete from cs_active_job; insert into cs_active_job(job_id) values(v_job_id); begin insert into cs_jobs(job_id, start_stamp) values(v_job_id, sysdate); exception when dup_val_on_index then null; -- don't worry if it already exists end; commit; end; / show errors Procedures like this can be easily converted into PostgreSQL functions returning an INTEGER. This procedure in particular is interesting because it can teach us some things: There is no pragma statement in PostgreSQL. If you do a LOCK TABLE in PL/pgSQL, the lock will not be released until the calling transaction is finished. You also cannot have transactions in PL/pgSQL procedures. The entire function (and other functions called from therein) is executed in a transaction and PostgreSQL rolls back the results if something goes wrong. Therefore only one BEGIN statement is allowed. The exception when would have to be replaced by an IF statement. So let's see one of the ways we could port this procedure to PL/pgSQL: drop function cs_create_job(integer); create function cs_create_job(integer) returns integer as ' declare v_job_id alias for $1; a_running_job_count integer; a_num integer; -- pragma autonomous_transaction; begin lock table cs_jobs in exclusive mode; select count(*) into a_running_job_count from cs_jobs where end_stamp is null; if a_running_job_count > 0 then -- commit; -- free lock raise exception ''Unable to create a new job: a job is currently running.''; end if; delete from cs_active_job; insert into cs_active_job(job_id) values(v_job_id); SELECT count(*) into a_num FROM cs_jobs WHERE job_id=v_job_id; IF NOT FOUND THEN -- If nothing was returned in the last query -- This job is not in the table so lets insert it. insert into cs_jobs(job_id, start_stamp) values(v_job_id, sysdate()); return 1; ELSE raise NOTICE ''Job already running.''; END IF; return 0; end; ' language 'plpgsql'; Notice how you can raise notices (or errors) in PL/pgSQL. I haven't done much with packages myself, so if there are mistakes here, please let me know. Packages are a way Oracle gives you to encapsulate PL/SQL statements and functions into one entity, like Java classes, where you define methods and objects. You can access these objects/methods with a . (dot). Here is an example of an Oracle package from ACS 4 (the ArsDigita Community System): create or replace package body acs as function add_user ( user_id in users.user_id%TYPE default null, object_type in acs_objects.object_type%TYPE default 'user', creation_date in acs_objects.creation_date%TYPE default sysdate, creation_user in acs_objects.creation_user%TYPE default null, creation_ip in acs_objects.creation_ip%TYPE default null, ... ) return users.user_id%TYPE is v_user_id users.user_id%TYPE; v_rel_id membership_rels.rel_id%TYPE; begin v_user_id := acs_user.new (user_id, object_type, creation_date, creation_user, creation_ip, email, ... return v_user_id; end; end acs; / show errors We port this to PostgreSQL by creating the different objects of the Oracle package as functions with a standard naming convention. We have to pay attention to some other details, like the lack of default parameters in PostgreSQL functions. The above package would become something like this: CREATE FUNCTION acs__add_user(integer,integer,varchar,datetime,integer,integer,...) RETURNS integer AS ' DECLARE user_id ALIAS FOR $1; object_type ALIAS FOR $2; creation_date ALIAS FOR $3; creation_user ALIAS FOR $4; creation_ip ALIAS FOR $5; ... v_user_id users.user_id%TYPE; v_rel_id membership_rels.rel_id%TYPE; BEGIN v_user_id := acs_user__new(user_id,object_type,creation_date,creation_user,creation_ip, ...); ... return v_user_id; END; ' LANGUAGE 'plpgsql'; The PostgreSQL version of EXECUTE works nicely, but you have to remember to use quote_literal(TEXT) and quote_string(TEXT) as described in . Constructs of the type EXECUTE ''SELECT * from $1''; will not work unless you use these functions. PostgreSQL gives you two function creation modifiers to optimize execution: iscachable (function always returns the same result when given the same arguments) and isstrict (function returns NULL if any argument is NULL). Consult the CREATE FUNCTION reference for details. To make use of these optimization attributes, you have to use the WITH modifier in your CREATE FUNCTION statement. Something like: CREATE FUNCTION foo(...) RETURNS integer AS ' ... ' LANGUAGE 'plpgsql' WITH (isstrict, iscachable); This function should probably be integrated into the core. -- -- instr functions that mimic Oracle's counterpart -- Syntax: instr(string1,string2,[n],[m]) where [] denotes optional params. -- -- Searches string1 beginning at the nth character for the mth -- occurrence of string2. If n is negative, search backwards. If m is -- not passed, assume 1 (search starts at first character). -- -- by Roberto Mello (rmello@fslc.usu.edu) -- modified by Robert Gaszewski (graszew@poland.com) -- Licensed under the GPL v2 or later. -- DROP FUNCTION instr(varchar,varchar); CREATE FUNCTION instr(varchar,varchar) RETURNS integer AS ' DECLARE pos integer; BEGIN pos:= instr($1,$2,1); RETURN pos; END; ' language 'plpgsql'; DROP FUNCTION instr(varchar,varchar,integer); CREATE FUNCTION instr(varchar,varchar,integer) RETURNS integer AS ' DECLARE string ALIAS FOR $1; string_to_search ALIAS FOR $2; beg_index ALIAS FOR $3; pos integer NOT NULL DEFAULT 0; temp_str varchar; beg integer; length integer; ss_length integer; BEGIN IF beg_index > 0 THEN temp_str := substring(string FROM beg_index); pos := position(string_to_search IN temp_str); IF pos = 0 THEN RETURN 0; ELSE RETURN pos + beg_index - 1; END IF; ELSE ss_length := char_length(string_to_search); length := char_length(string); beg := length + beg_index - ss_length + 2; WHILE beg > 0 LOOP temp_str := substring(string FROM beg FOR ss_length); pos := position(string_to_search IN temp_str); IF pos > 0 THEN RETURN beg; END IF; beg := beg - 1; END LOOP; RETURN 0; END IF; END; ' language 'plpgsql'; -- -- Written by Robert Gaszewski (graszew@poland.com) -- Licensed under the GPL v2 or later. -- DROP FUNCTION instr(varchar,varchar,integer,integer); CREATE FUNCTION instr(varchar,varchar,integer,integer) RETURNS integer AS ' DECLARE string ALIAS FOR $1; string_to_search ALIAS FOR $2; beg_index ALIAS FOR $3; occur_index ALIAS FOR $4; pos integer NOT NULL DEFAULT 0; occur_number integer NOT NULL DEFAULT 0; temp_str varchar; beg integer; i integer; length integer; ss_length integer; BEGIN IF beg_index > 0 THEN beg := beg_index; temp_str := substring(string FROM beg_index); FOR i IN 1..occur_index LOOP pos := position(string_to_search IN temp_str); IF i = 1 THEN beg := beg + pos - 1; ELSE beg := beg + pos; END IF; temp_str := substring(string FROM beg + 1); END LOOP; IF pos = 0 THEN RETURN 0; ELSE RETURN beg; END IF; ELSE ss_length := char_length(string_to_search); length := char_length(string); beg := length + beg_index - ss_length + 2; WHILE beg > 0 LOOP temp_str := substring(string FROM beg FOR ss_length); pos := position(string_to_search IN temp_str); IF pos > 0 THEN occur_number := occur_number + 1; IF occur_number = occur_index THEN RETURN beg; END IF; END IF; beg := beg - 1; END LOOP; RETURN 0; END IF; END; ' language 'plpgsql';