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Postgres FD Implementation
Commit 1c63587f authored by Peter Eisentraut's avatar Peter Eisentraut
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Clean up markup, add description of contrib/array operators by Joel Burton 

<jburton@scw.org>.
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doc/src/sgml/array.sgml
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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/array.sgml,v 1.9 2001/01/13 23:58:55 petere Exp $
-->
<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 columns of a table
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 table:
<ProgramListing>
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/array.sgml,v 1.10 2001/01/26 23:40:39 petere Exp $ -->
<chapter id="arrays">
<title>Arrays</title>
<para>
<productname>Postgres</productname> allows columns of a table 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 table:
<programlisting>
CREATE TABLE sal_emp (
name text,
pay_by_quarter int4[],
pay_by_quarter integer[],
schedule text[][]
);
</ProgramListing>
</Para>
<Para>
The above query will create a table 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>INSERT</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>
</programlisting>
The above query will create a table named
<structname>sal_emp</structname> with a <type>text</type> string
(<structfield>name</structfield>), a one-dimensional array of type
<type>integer</type> (<structfield>pay_by_quarter</structfield>),
which shall represent the employee's salary by quarter, and a
two-dimensional array of <type>text</type>
(<structfield>schedule</structfield>), which represents the
employee's weekly schedule.
</para>
<para>
Now we do some <command>INSERT</command>s; note that when appending
to an array, we enclose the values within braces and separate them
by commas. If you know C, this is not unlike the syntax for
initializing structures.
<programlisting>
INSERT INTO sal_emp
VALUES ('Bill',
'{10000, 10000, 10000, 10000}',
......@@ -49,32 +41,34 @@ INSERT INTO sal_emp
VALUES ('Carol',
'{20000, 25000, 25000, 25000}',
'{{"talk", "consult"}, {"meeting"}}');
</ProgramListing>
</programlisting>
</para>
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:
<para>
Now, we can run some queries on <structname>sal_emp</structname>.
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>
<programlisting>
SELECT name FROM sal_emp WHERE pay_by_quarter[1] &lt;&gt; pay_by_quarter[2];
name
-------
Carol
(1 row)
</ProgramListing>
</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>
<productname>Postgres</productname> uses the
<quote>one-based</quote> numbering convention for arrays, that is,
an array of n elements starts with <literal>array[1]</literal> and
ends with <literal>array[n]</literal>.
</para>
<Para>
This query retrieves the third quarter pay of all
employees:
<para>
This query retrieves the third quarter pay of all employees:
<ProgramListing>
<programlisting>
SELECT pay_by_quarter[3] FROM sal_emp;
pay_by_quarter
......@@ -82,110 +76,161 @@ SELECT pay_by_quarter[3] FROM sal_emp;
10000
25000
(2 rows)
</ProgramListing>
</Para>
</programlisting>
</para>
<Para>
<para>
We can also access arbitrary rectangular slices of an array, or
subarrays. An array slice is denoted by writing
<replaceable>lower subscript</replaceable> <literal>:</literal>
<replaceable>upper subscript</replaceable> for one or more array
dimensions. This query retrieves the first item on
Bill's schedule for the first two days of the week:
<literal><replaceable>lower subscript</replaceable> :
<replaceable>upper subscript</replaceable></literal> 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>
<programlisting>
SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';
schedule
--------------------
{{"meeting"},{""}}
(1 row)
</ProgramListing>
</programlisting>
We could also have written
<ProgramListing>
<programlisting>
SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';
</ProgramListing>
</programlisting>
with the same result. An array subscripting operation is taken to
represent an array slice if any of the subscripts are written in
the form <replaceable>lower</replaceable> <literal>:</literal>
<replaceable>upper</replaceable>. A lower bound of 1 is assumed
for any subscript where only one value is specified.
</Para>
represent an array slice if any of the subscripts are written in the
form <replaceable>lower</replaceable> <literal>:</literal>
<replaceable>upper</replaceable>. A lower bound of 1 is assumed for
any subscript where only one value is specified.
</para>
<Para>
<para>
An array value can be replaced completely:
<ProgramListing>
<programlisting>
UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}'
WHERE name = 'Carol';
</ProgramListing>
</programlisting>
or updated at a single element:
<ProgramListing>
<programlisting>
UPDATE sal_emp SET pay_by_quarter[4] = 15000
WHERE name = 'Bill';
</ProgramListing>
</programListing>
or updated in a slice:
<ProgramListing>
<programlisting>
UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}'
WHERE name = 'Carol';
</ProgramListing>
</Para>
</programlisting>
</para>
<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.
For example, if an array value currently has 4 elements, it will
have five elements after an update that assigns to array[5].
Currently, enlargement in this fashion is only
allowed for one-dimensional arrays, not multidimensional arrays.
</Para>
<Para>
The syntax for CREATE TABLE allows fixed-length arrays to be
defined:
<ProgramListing>
to or overlaps the data already present. For example, if an array
value currently has 4 elements, it will have five elements after an
update that assigns to array[5]. Currently, enlargement in this
fashion is only allowed for one-dimensional arrays, not
multidimensional arrays.
</para>
<para>
The syntax for <command>CREATE TABLE</command> allows fixed-length
arrays to be defined:
<programlisting>
CREATE TABLE tictactoe (
squares int4[3][3]
squares integer[3][3]
);
</ProgramListing>
</programlisting>
However, the current implementation does not enforce the array
size limits --- the behavior is the same as for arrays of
unspecified length.
</Para>
However, the current implementation does not enforce the array size
limits --- the behavior is the same as for arrays of unspecified
length.
</para>
<Para>
Actually, the current implementation doesn't enforce the declared
number of dimensions either. Arrays of a particular base type
are all considered to be of the same type, regardless of size or
number of dimensions.
</Para>
<para>
Actually, the current implementation does not enforce the declared
number of dimensions either. Arrays of a particular base type are
all considered to be of the same type, regardless of size or number
of dimensions.
</para>
<Para>
The current dimensions of any array value can be retrieved with
the <function>array_dims</function> function:
<para>
The current dimensions of any array value can be retrieved with the
<function>array_dims</function> function:
<ProgramListing>
<programlisting>
SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol';
array_dims
------------
[1:2][1:1]
(1 row)
</ProgramListing>
</programlisting>
<function>array_dims</function> produces a <type>text</type> result,
which is convenient for people to read but perhaps not so convenient
for programs.
</Para>
</Chapter>
</para>
<para>
To search for a value in an array, you must check each value of the
array. This can be done by hand (if you know the size of the array):
<programlisting>
SELECT * FROM sal_emp WHERE pay_by_quarter[1] = 10000 OR
pay_by_quarter[2] = 10000 OR
pay_by_quarter[3] = 10000 OR
pay_by_quarter[4] = 10000;
</programlisting>
However, this quickly becomes tedious for large arrays, and is not
helpful if the size of the array is unknown. Although it is not part
of the primary <productname>PostgreSQL</productname> distribution,
in the contributions directory, there is an extension to
<productname>PostgreSQL</productname> that defines new functions and
operators for iterating over array values. Using this, the above
query could be:
<programlisting>
SELECT * FROM sal_emp WHERE pay_by_quarter[1:4] *= 10000;
</programlisting>
To search the entire array (not just specified columns), you could
use:
<programlisting>
SELECT * FROM sal_emp WHERE pay_by_quarter *= 10000;
</programlisting>
In addition, you could find rows where the array had all values
equal to 10 000 with:
<programlisting>
SELECT * FROM sal_emp WHERE pay_by_quarter **= 10000;
</programlisting>
To install this optional module, look in the
<filename>contrib/array</filename> directory of the
<productname>PostgreSQL</productname> source distribution.
</para>
<tip>
<para>
Arrays are not lists; using arrays in the manner described in the
previous paragraph is often a sign of database misdesign. The
array field should generally be split off into a separate table.
Tables can obviously be searched easily.
</para>
</tip>
</chapter>
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