Skip to content

P
Postgres FD Implementation
Commit 1c63587f authored by Peter Eisentraut's avatar Peter Eisentraut
Browse files
Options

Clean up markup, add description of contrib/array operators by Joel Burton 

<jburton@scw.org>.
parent b78d1bed
Show whitespace changes
Inline Side-by-side
Showing with 170 additions and 125 deletions
doc/src/sgml/array.sgml
View file @ 1c63587f
<!-- <!-- $Header: /cvsroot/pgsql/doc/src/sgml/array.sgml,v 1.10 2001/01/26 23:40:39 petere Exp $ -->
$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>
<Chapter Id="arrays">
<Title>Arrays</Title> <para>
<productname>Postgres</productname> allows columns of a table to be
<Para> defined as variable-length multi-dimensional arrays. Arrays of any
<Note> built-in type or user-defined type can be created. To illustrate
<Para> their use, we create this table:
This must become a chapter on array behavior. Volunteers? - thomas 1998-01-12 <programlisting>
</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>
CREATE TABLE sal_emp ( CREATE TABLE sal_emp (
name text, name text,
pay_by_quarter int4[], pay_by_quarter integer[],
schedule text[][] schedule text[][]
); );
</ProgramListing> </programlisting>
</Para> The above query will create a table named
<structname>sal_emp</structname> with a <type>text</type> string
<Para> (<structfield>name</structfield>), a one-dimensional array of type
The above query will create a table named <FirstTerm>sal_emp</FirstTerm> with <type>integer</type> (<structfield>pay_by_quarter</structfield>),
a <FirstTerm>text</FirstTerm> string (name), a one-dimensional array of <FirstTerm>int4</FirstTerm> which shall represent the employee's salary by quarter, and a
(pay_by_quarter), which represents the employee's two-dimensional array of <type>text</type>
salary by quarter, and a two-dimensional array of <FirstTerm>text</FirstTerm> (<structfield>schedule</structfield>), which represents the
(schedule), which represents the employee's weekly employee's weekly schedule.
schedule. Now we do some <FirstTerm>INSERT</FirstTerm>s; note that when </para>
appending to an array, we enclose the values within
braces and separate them by commas. If you know <FirstTerm>C</FirstTerm>, <para>
this is not unlike the syntax for initializing structures. Now we do some <command>INSERT</command>s; note that when appending
to an array, we enclose the values within braces and separate them
<ProgramListing> by commas. If you know C, this is not unlike the syntax for
initializing structures.
<programlisting>
INSERT INTO sal_emp INSERT INTO sal_emp
VALUES ('Bill', VALUES ('Bill',
'{10000, 10000, 10000, 10000}', '{10000, 10000, 10000, 10000}',
...@@ -49,32 +41,34 @@ INSERT INTO sal_emp ...@@ -49,32 +41,34 @@ INSERT INTO sal_emp
VALUES ('Carol', VALUES ('Carol',
'{20000, 25000, 25000, 25000}', '{20000, 25000, 25000, 25000}',
'{{"talk", "consult"}, {"meeting"}}'); '{{"talk", "consult"}, {"meeting"}}');
</ProgramListing> </programlisting>
</para>
Now, we can run some queries on sal_emp. First, we <para>
show how to access a single element of an array at a Now, we can run some queries on <structname>sal_emp</structname>.
time. This query retrieves the names of the employees First, we show how to access a single element of an array at a time.
whose pay changed in the second quarter: 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]; SELECT name FROM sal_emp WHERE pay_by_quarter[1] &lt;&gt; pay_by_quarter[2];
name name
------- -------
Carol Carol
(1 row) (1 row)
</ProgramListing> </programlisting>
<ProductName>Postgres</ProductName> uses the "one-based" numbering <productname>Postgres</productname> uses the
convention for arrays --- that is, an array of n elements starts with <quote>one-based</quote> numbering convention for arrays, that is,
array[1] and ends with array[n]. an array of n elements starts with <literal>array[1]</literal> and
</Para> ends with <literal>array[n]</literal>.
</para>
<Para> <para>
This query retrieves the third quarter pay of all This query retrieves the third quarter pay of all employees:
employees:
<ProgramListing> <programlisting>
SELECT pay_by_quarter[3] FROM sal_emp; SELECT pay_by_quarter[3] FROM sal_emp;
pay_by_quarter pay_by_quarter
...@@ -82,110 +76,161 @@ SELECT pay_by_quarter[3] FROM sal_emp; ...@@ -82,110 +76,161 @@ SELECT pay_by_quarter[3] FROM sal_emp;
10000 10000
25000 25000
(2 rows) (2 rows)
</ProgramListing> </programlisting>
</Para> </para>
<Para> <para>
We can also access arbitrary rectangular slices of an array, or We can also access arbitrary rectangular slices of an array, or
subarrays. An array slice is denoted by writing subarrays. An array slice is denoted by writing
<replaceable>lower subscript</replaceable> <literal>:</literal> <literal><replaceable>lower subscript</replaceable> :
<replaceable>upper subscript</replaceable> for one or more array <replaceable>upper subscript</replaceable></literal> for one or more
dimensions. This query retrieves the first item on array dimensions. This query retrieves the first item on Bill's
Bill's schedule for the first two days of the week: schedule for the first two days of the week:
<ProgramListing> <programlisting>
SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill'; SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';
schedule schedule
-------------------- --------------------
{{"meeting"},{""}} {{"meeting"},{""}}
(1 row) (1 row)
</ProgramListing> </programlisting>
We could also have written We could also have written
<ProgramListing> <programlisting>
SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill'; SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';
</ProgramListing> </programlisting>
with the same result. An array subscripting operation is taken to with the same result. An array subscripting operation is taken to
represent an array slice if any of the subscripts are written in represent an array slice if any of the subscripts are written in the
the form <replaceable>lower</replaceable> <literal>:</literal> form <replaceable>lower</replaceable> <literal>:</literal>
<replaceable>upper</replaceable>. A lower bound of 1 is assumed <replaceable>upper</replaceable>. A lower bound of 1 is assumed for
for any subscript where only one value is specified. any subscript where only one value is specified.
</Para> </para>
<Para> <para>
An array value can be replaced completely: An array value can be replaced completely:
<ProgramListing> <programlisting>
UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}' UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}'
WHERE name = 'Carol'; WHERE name = 'Carol';
</ProgramListing> </programlisting>
or updated at a single element: or updated at a single element:
<ProgramListing> <programlisting>
UPDATE sal_emp SET pay_by_quarter[4] = 15000 UPDATE sal_emp SET pay_by_quarter[4] = 15000
WHERE name = 'Bill'; WHERE name = 'Bill';
</ProgramListing> </programListing>
or updated in a slice: or updated in a slice:
<ProgramListing> <programlisting>
UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}' UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}'
WHERE name = 'Carol'; WHERE name = 'Carol';
</ProgramListing> </programlisting>
</Para> </para>
<Para> <para>
An array can be enlarged by assigning to an element adjacent to An array can be enlarged by assigning to an element adjacent to
those already present, or by assigning to a slice that is adjacent those already present, or by assigning to a slice that is adjacent
to or overlaps the data already present. to or overlaps the data already present. For example, if an array
For example, if an array value currently has 4 elements, it will value currently has 4 elements, it will have five elements after an
have five elements after an update that assigns to array[5]. update that assigns to array[5]. Currently, enlargement in this
Currently, enlargement in this fashion is only fashion is only allowed for one-dimensional arrays, not
allowed for one-dimensional arrays, not multidimensional arrays. multidimensional arrays.
</Para> </para>
<Para> <para>
The syntax for CREATE TABLE allows fixed-length arrays to be The syntax for <command>CREATE TABLE</command> allows fixed-length
defined: arrays to be defined:
<ProgramListing> <programlisting>
CREATE TABLE tictactoe ( CREATE TABLE tictactoe (
squares int4[3][3] squares integer[3][3]
); );
</ProgramListing> </programlisting>
However, the current implementation does not enforce the array However, the current implementation does not enforce the array size
size limits --- the behavior is the same as for arrays of limits --- the behavior is the same as for arrays of unspecified
unspecified length. length.
</Para> </para>
<Para> <para>
Actually, the current implementation doesn't enforce the declared Actually, the current implementation does not enforce the declared
number of dimensions either. Arrays of a particular base type number of dimensions either. Arrays of a particular base type are
are all considered to be of the same type, regardless of size or all considered to be of the same type, regardless of size or number
number of dimensions. of dimensions.
</Para> </para>
<Para> <para>
The current dimensions of any array value can be retrieved with The current dimensions of any array value can be retrieved with the
the <function>array_dims</function> function: <function>array_dims</function> function:
<ProgramListing> <programlisting>
SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol'; SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol';
array_dims array_dims
------------ ------------
[1:2][1:1] [1:2][1:1]
(1 row) (1 row)
</ProgramListing> </programlisting>
<function>array_dims</function> produces a <type>text</type> result, <function>array_dims</function> produces a <type>text</type> result,
which is convenient for people to read but perhaps not so convenient which is convenient for people to read but perhaps not so convenient
for programs. for programs.
</Para> </para>
</Chapter> <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>
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment