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<Chapter Id="arrays">
<Title>Arrays</Title>
<Para>
<Note>
<Para>
This must become a chapter on array behavior. Volunteers? - thomas 1998-01-12
</Para>
</Note>
</Para>
<Para>
<ProductName>Postgres</ProductName> allows attributes of a class
to be defined as variable-length multi-dimensional
arrays. Arrays of any built-in type or user-defined type
can be created. To illustrate their use, we create this class:
<ProgramListing>
CREATE TABLE sal_emp (
name text,
pay_by_quarter int4[],
schedule text[][]
);
</ProgramListing>
</Para>
<Para>
The above query will create a class named <FirstTerm>sal_emp</FirstTerm> with
a <FirstTerm>text</FirstTerm> string (name), a one-dimensional array of <FirstTerm>int4</FirstTerm>
(pay_by_quarter), which represents the employee's
salary by quarter, and a two-dimensional array of <FirstTerm>text</FirstTerm>
(schedule), which represents the employee's weekly
schedule. Now we do some <FirstTerm>INSERTS</FirstTerm>s; note that when
appending to an array, we enclose the values within
braces and separate them by commas. If you know <FirstTerm>C</FirstTerm>,
this is not unlike the syntax for initializing structures.
<ProgramListing>
INSERT INTO sal_emp
VALUES ('Bill',
'{10000, 10000, 10000, 10000}',
'{{"meeting", "lunch"}, {}}');
INSERT INTO sal_emp
VALUES ('Carol',
'{20000, 25000, 25000, 25000}',
'{{"talk", "consult"}, {"meeting"}}');
</ProgramListing>
Now, we can run some queries on sal_emp. First, we
show how to access a single element of an array at a
time. This query retrieves the names of the employees
whose pay changed in the second quarter:
<ProgramListing>
SELECT name FROM sal_emp WHERE pay_by_quarter[1] <> pay_by_quarter[2];
name
-------
Carol
(1 row)
</ProgramListing>
<ProductName>Postgres</ProductName> uses the "one-based" numbering
convention for arrays --- that is, an array of n elements starts with
array[1] and ends with array[n].
</Para>
<Para>
This query retrieves the third quarter pay of all
employees:
<ProgramListing>
SELECT pay_by_quarter[3] FROM sal_emp;
pay_by_quarter
----------------
10000
25000
(2 rows)
</ProgramListing>
</Para>
<Para>
We can also access arbitrary slices of an array, or
subarrays. An array slice is denoted by writing
"lower subscript : upper subscript" for one or more array
dimensions. This query retrieves the first item on
Bill's schedule for the first two days of the week:
<ProgramListing>
SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';
schedule
--------------------
{{"meeting"},{""}}
(1 row)
</ProgramListing>
We could also have written
<ProgramListing>
SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';
</ProgramListing>
with the same result.
</Para>
<Para>
An array value can be replaced completely:
<ProgramListing>
UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}'
WHERE name = 'Carol';
</ProgramListing>
or updated at a single entry:
<ProgramListing>
UPDATE sal_emp SET pay_by_quarter[4] = 15000
WHERE name = 'Bill';
</ProgramListing>
or updated in a slice:
<ProgramListing>
UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}'
WHERE name = 'Carol';
</ProgramListing>
</Para>
<Para>
An array can be enlarged by assigning to an element adjacent to
those already present, or by assigning to a slice that is adjacent
to or overlaps the data already present. Currently, this is only
allowed for one-dimensional arrays, not multidimensional arrays.
For example, if an array value currently has 4 elements, it will
have five elements after an update that assigns to array[5].
</Para>
<Para>
The syntax for CREATE TABLE allows fixed-length arrays to be
defined:
<ProgramListing>
CREATE TABLE tictactoe (
squares int4[3][3]
);
</ProgramListing>
However, the current implementation does not enforce the array
size limits --- the behavior is the same as for arrays of
unspecified length.
</Para>
</Chapter>
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