PL/Tcl Tcl PL/Tcl is a loadable procedural language for the PostgreSQL database system that enables the Tcl language to be used to write functions and trigger procedures. PL/Tcl offers most of the capabilities a function writer has in the C language, with a few restrictions, and with the addition of the powerful string processing libraries that are available for Tcl. One compelling good restriction is that everything is executed from within the safety of the context of a Tcl interpreter. In addition to the limited command set of safe Tcl, only a few commands are available to access the database via SPI and to raise messages via elog(). PL/Tcl provides no way to access internals of the database server or to gain OS-level access under the permissions of the PostgreSQL server process, as a C function can do. Thus, unprivileged database users may be trusted to use this language; it does not give them unlimited authority. The other notable implementation restriction is that Tcl functions may not be used to create input/output functions for new data types. Sometimes it is desirable to write Tcl functions that are not restricted to safe Tcl. For example, one might want a Tcl function that sends email. To handle these cases, there is a variant of PL/Tcl called PL/TclU (for untrusted Tcl). This is the exact same language except that a full Tcl interpreter is used. If PL/TclU is used, it must be installed as an untrusted procedural language so that only database superusers can create functions in it. The writer of a PL/TclU function must take care that the function cannot be used to do anything unwanted, since it will be able to do anything that could be done by a user logged in as the database administrator. The shared object code for the PL/Tcl and PL/TclU call handlers is automatically built and installed in the PostgreSQL library directory if Tcl support is specified in the configuration step of the installation procedure. To install PL/Tcl and/or PL/TclU in a particular database, use the createlang program, for example createlang pltcl dbname or createlang pltclu dbname. To create a function in the PL/Tcl language, use the standard syntax: CREATE FUNCTION funcname (argument-types) RETURNS return-type AS $$ # PL/Tcl function body $$ LANGUAGE pltcl; PL/TclU is the same, except that the language has to be specified as pltclu. The body of the function is simply a piece of Tcl script. When the function is called, the argument values are passed as variables $1 ... $n to the Tcl script. The result is returned from the Tcl code in the usual way, with a return statement. For example, a function returning the greater of two integer values could be defined as: CREATE FUNCTION tcl_max(integer, integer) RETURNS integer AS $$ if {$1 > $2} {return $1} return $2 $$ LANGUAGE pltcl STRICT; Note the clause STRICT, which saves us from having to think about null input values: if a null value is passed, the function will not be called at all, but will just return a null result automatically. In a nonstrict function, if the actual value of an argument is null, the corresponding $n variable will be set to an empty string. To detect whether a particular argument is null, use the function argisnull. For example, suppose that we wanted tcl_max with one null and one nonnull argument to return the nonnull argument, rather than null: CREATE FUNCTION tcl_max(integer, integer) RETURNS integer AS $$ if {[argisnull 1]} { if {[argisnull 2]} { return_null } return $2 } if {[argisnull 2]} { return $1 } if {$1 > $2} {return $1} return $2 $$ LANGUAGE pltcl; As shown above, to return a null value from a PL/Tcl function, execute return_null. This can be done whether the function is strict or not. Composite-type arguments are passed to the function as Tcl arrays. The element names of the array are the attribute names of the composite type. If an attribute in the passed row has the null value, it will not appear in the array. Here is an example: CREATE TABLE employee ( name text, salary integer, age integer ); CREATE FUNCTION overpaid(employee) RETURNS boolean AS $$ if {200000.0 < $1(salary)} { return "t" } if {$1(age) < 30 && 100000.0 < $1(salary)} { return "t" } return "f" $$ LANGUAGE pltcl; There is currently no support for returning a composite-type result value, nor for returning sets. PL/Tcl does not currently have full support for domain types: it treats a domain the same as the underlying scalar type. This means that constraints associated with the domain will not be enforced. This is not an issue for function arguments, but it is a hazard if you declare a PL/Tcl function as returning a domain type. The argument values supplied to a PL/Tcl function's code are simply the input arguments converted to text form (just as if they had been displayed by a SELECT statement). Conversely, the return command will accept any string that is acceptable input format for the function's declared return type. So, within the PL/Tcl function, all values are just text strings. global data in PL/Tcl Sometimes it is useful to have some global data that is held between two calls to a function or is shared between different functions. This is easily done since all PL/Tcl functions executed in one session share the same safe Tcl interpreter. So, any global Tcl variable is accessible to all PL/Tcl function calls and will persist for the duration of the SQL session. (Note that PL/TclU functions likewise share global data, but they are in a different Tcl interpreter and cannot communicate with PL/Tcl functions.) To help protect PL/Tcl functions from unintentionally interfering with each other, a global array is made available to each function via the upvar command. The global name of this variable is the function's internal name, and the local name is GD. It is recommended that GD be used for persistent private data of a function. Use regular Tcl global variables only for values that you specifically intend to be shared among multiple functions. An example of using GD appears in the spi_execp example below. The following commands are available to access the database from the body of a PL/Tcl function: spi_exec -count n -array name command loop-body Executes an SQL command given as a string. An error in the command causes an error to be raised. Otherwise, the return value of spi_exec is the number of rows processed (selected, inserted, updated, or deleted) by the command, or zero if the command is a utility statement. In addition, if the command is a SELECT statement, the values of the selected columns are placed in Tcl variables as described below. The optional -count value tells spi_exec the maximum number of rows to process in the command. The effect of this is comparable to setting up a query as a cursor and then saying FETCH n. If the command is a SELECT statement, the values of the result columns are placed into Tcl variables named after the columns. If the -array option is given, the column values are instead stored into the named associative array, with the column names used as array indexes. If the command is a SELECT statement and no loop-body script is given, then only the first row of results are stored into Tcl variables; remaining rows, if any, are ignored. No storing occurs if the query returns no rows. (This case can be detected by checking the result of spi_exec.) For example, spi_exec "SELECT count(*) AS cnt FROM pg_proc" will set the Tcl variable $cnt to the number of rows in the pg_proc system catalog. If the optional loop-body argument is given, it is a piece of Tcl script that is executed once for each row in the query result. (loop-body is ignored if the given command is not a SELECT.) The values of the current row's columns are stored into Tcl variables before each iteration. For example, spi_exec -array C "SELECT * FROM pg_class" { elog DEBUG "have table $C(relname)" } will print a log message for every row of pg_class. This feature works similarly to other Tcl looping constructs; in particular continue and break work in the usual way inside the loop body. If a column of a query result is null, the target variable for it is unset rather than being set. spi_prepare query typelist Prepares and saves a query plan for later execution. The saved plan will be retained for the life of the current session.preparing a queryin PL/Tcl The query may use parameters, that is, placeholders for values to be supplied whenever the plan is actually executed. In the query string, refer to parameters by the symbols $1 ... $n. If the query uses parameters, the names of the parameter types must be given as a Tcl list. (Write an empty list for typelist if no parameters are used.) Presently, the parameter types must be identified by the internal type names shown in the system table pg_type; for example int4 not integer. The return value from spi_prepare is a query ID to be used in subsequent calls to spi_execp. See spi_execp for an example. spi_execp -count n -array name -nulls string queryid value-list loop-body Executes a query previously prepared with spi_prepare. queryid is the ID returned by spi_prepare. If the query references parameters, a value-list must be supplied. This is a Tcl list of actual values for the parameters. The list must be the same length as the parameter type list previously given to spi_prepare. Omit value-list if the query has no parameters. The optional value for -nulls is a string of spaces and 'n' characters telling spi_execp which of the parameters are null values. If given, it must have exactly the same length as the value-list. If it is not given, all the parameter values are nonnull. Except for the way in which the query and its parameters are specified, spi_execp works just like spi_exec. The -count, -array, and loop-body options are the same, and so is the result value. Here's an example of a PL/Tcl function using a prepared plan: CREATE FUNCTION t1_count(integer, integer) RETURNS integer AS $$ if {![ info exists GD(plan) ]} { # prepare the saved plan on the first call set GD(plan) [ spi_prepare \ "SELECT count(*) AS cnt FROM t1 WHERE num >= \$1 AND num <= \$2" \ [ list int4 int4 ] ] } spi_execp -count 1 $GD(plan) [ list $1 $2 ] return $cnt $$ LANGUAGE pltcl; We need backslashes inside the query string given to spi_prepare to ensure that the $n markers will be passed through to spi_prepare as-is, and not replaced by Tcl variable substitution. spi_lastoid spi_lastoid Returns the OID of the row inserted by the last spi_exec or spi_execp, if the command was a single-row INSERT and the modified table contained OIDs. (If not, you get zero.) quote string Doubles all occurrences of single quote and backslash characters in the given string. This may be used to safely quote strings that are to be inserted into SQL commands given to spi_exec or spi_prepare. For example, think about an SQL command string like "SELECT '$val' AS ret" where the Tcl variable val actually contains doesn't. This would result in the final command string SELECT 'doesn't' AS ret which would cause a parse error during spi_exec or spi_prepare. To work properly, the submitted command should contain SELECT 'doesn''t' AS ret which can be formed in PL/Tcl using "SELECT '[ quote $val ]' AS ret" One advantage of spi_execp is that you don't have to quote parameter values like this, since the parameters are never parsed as part of an SQL command string. elog in PL/Tcl elog level msg Emits a log or error message. Possible levels are DEBUG, LOG, INFO, NOTICE, WARNING, ERROR, and FATAL. ERROR raises an error condition; if this is not trapped by the surrounding Tcl code, the error propagates out to the calling query, causing the current transaction or subtransaction to be aborted. This is effectively the same as the Tcl error command. FATAL aborts the transaction and causes the current session to shut down. (There is probably no good reason to use this error level in PL/Tcl functions, but it's provided for completeness.) The other levels only generate messages of different priority levels. Whether messages of a particular priority are reported to the client, written to the server log, or both is controlled by the and configuration variables. See for more information. trigger in PL/Tcl Trigger procedures can be written in PL/Tcl. PostgreSQL requires that a procedure that is to be called as a trigger must be declared as a function with no arguments and a return type of trigger. The information from the trigger manager is passed to the procedure body in the following variables: $TG_name The name of the trigger from the CREATE TRIGGER statement. $TG_relid The object ID of the table that caused the trigger procedure to be invoked. $TG_relatts A Tcl list of the table column names, prefixed with an empty list element. So looking up a column name in the list with Tcl's lsearch command returns the element's number starting with 1 for the first column, the same way the columns are customarily numbered in PostgreSQL. (Empty list elements also appear in the positions of columns that have been dropped, so that the attribute numbering is correct for columns to their right.) $TG_when The string BEFORE or AFTER depending on the type of trigger call. $TG_level The string ROW or STATEMENT depending on the type of trigger call. $TG_op The string INSERT, UPDATE, or DELETE depending on the type of trigger call. $NEW An associative array containing the values of the new table row for INSERT or UPDATE actions, or empty for DELETE. The array is indexed by column name. Columns that are null will not appear in the array. $OLD An associative array containing the values of the old table row for UPDATE or DELETE actions, or empty for INSERT. The array is indexed by column name. Columns that are null will not appear in the array. $args A Tcl list of the arguments to the procedure as given in the CREATE TRIGGER statement. These arguments are also accessible as $1 ... $n in the procedure body. The return value from a trigger procedure can be one of the strings OK or SKIP, or a list as returned by the array get Tcl command. If the return value is OK, the operation (INSERT/UPDATE/DELETE) that fired the trigger will proceed normally. SKIP tells the trigger manager to silently suppress the operation for this row. If a list is returned, it tells PL/Tcl to return a modified row to the trigger manager that will be inserted instead of the one given in $NEW. (This works for INSERT and UPDATE only.) Needless to say that all this is only meaningful when the trigger is BEFORE and FOR EACH ROW; otherwise the return value is ignored. Here's a little example trigger procedure that forces an integer value in a table to keep track of the number of updates that are performed on the row. For new rows inserted, the value is initialized to 0 and then incremented on every update operation. CREATE FUNCTION trigfunc_modcount() RETURNS trigger AS $$ switch $TG_op { INSERT { set NEW($1) 0 } UPDATE { set NEW($1) $OLD($1) incr NEW($1) } default { return OK } } return [array get NEW] $$ LANGUAGE pltcl; CREATE TABLE mytab (num integer, description text, modcnt integer); CREATE TRIGGER trig_mytab_modcount BEFORE INSERT OR UPDATE ON mytab FOR EACH ROW EXECUTE PROCEDURE trigfunc_modcount('modcnt'); Notice that the trigger procedure itself does not know the column name; that's supplied from the trigger arguments. This lets the trigger procedure be reused with different tables. PL/Tcl has support for autoloading Tcl code when used. It recognizes a special table, pltcl_modules, which is presumed to contain modules of Tcl code. If this table exists, the module unknown is fetched from the table and loaded into the Tcl interpreter immediately after creating the interpreter. While the unknown module could actually contain any initialization script you need, it normally defines a Tcl unknown procedure that is invoked whenever Tcl does not recognize an invoked procedure name. PL/Tcl's standard version of this procedure tries to find a module in pltcl_modules that will define the required procedure. If one is found, it is loaded into the interpreter, and then execution is allowed to proceed with the originally attempted procedure call. A secondary table pltcl_modfuncs provides an index of which functions are defined by which modules, so that the lookup is reasonably quick. The PostgreSQL distribution includes support scripts to maintain these tables: pltcl_loadmod, pltcl_listmod, pltcl_delmod, as well as source for the standard unknown module in share/unknown.pltcl. This module must be loaded into each database initially to support the autoloading mechanism. The tables pltcl_modules and pltcl_modfuncs must be readable by all, but it is wise to make them owned and writable only by the database administrator. In PostgreSQL, one and the same function name can be used for different functions as long as the number of arguments or their types differ. Tcl, however, requires all procedure names to be distinct. PL/Tcl deals with this by making the internal Tcl procedure names contain the object ID of the function from the system table pg_proc as part of their name. Thus, PostgreSQL functions with the same name and different argument types will be different Tcl procedures, too. This is not normally a concern for a PL/Tcl programmer, but it might be visible when debugging.