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636a939fe5
remove brain-dead rule that double quotes are needed if and only if the datatype is pass-by-reference; neither direction of the implication holds water. Instead, examine the actual data string to see if it contains any characters that force us to quote it. Add some documentation about quoting of array values, which was previously explained nowhere AFAICT.
283 lines
8.8 KiB
Text
283 lines
8.8 KiB
Text
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/array.sgml,v 1.18 2001/11/29 21:02:41 tgl Exp $ -->
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<chapter id="arrays">
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<title>Arrays</title>
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<indexterm>
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<primary>arrays</primary>
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</indexterm>
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<para>
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<productname>PostgreSQL</productname> allows columns of a table to be
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defined as variable-length multidimensional arrays. Arrays of any
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built-in type or user-defined type can be created. To illustrate
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their use, we create this table:
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<programlisting>
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CREATE TABLE sal_emp (
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name text,
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pay_by_quarter integer[],
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schedule text[][]
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);
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</programlisting>
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As shown, an array data type is named by appending square brackets
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(<literal>[]</>) to the data type name of the array elements.
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The above query will create a table named
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<structname>sal_emp</structname> with a <type>text</type> string
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(<structfield>name</structfield>), a one-dimensional array of type
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<type>integer</type> (<structfield>pay_by_quarter</structfield>),
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which represents the employee's salary by quarter, and a
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two-dimensional array of <type>text</type>
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(<structfield>schedule</structfield>), which represents the
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employee's weekly schedule.
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</para>
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<para>
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Now we do some <command>INSERT</command>s. Observe that to write an array
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value, we enclose the element values within curly braces and separate them
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by commas. If you know C, this is not unlike the syntax for
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initializing structures.
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<programlisting>
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INSERT INTO sal_emp
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VALUES ('Bill',
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'{10000, 10000, 10000, 10000}',
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'{{"meeting", "lunch"}, {}}');
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INSERT INTO sal_emp
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VALUES ('Carol',
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'{20000, 25000, 25000, 25000}',
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'{{"talk", "consult"}, {"meeting"}}');
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</programlisting>
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</para>
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<para>
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Now, we can run some queries on <structname>sal_emp</structname>.
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First, we show how to access a single element of an array at a time.
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This query retrieves the names of the employees whose pay changed in
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the second quarter:
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<programlisting>
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SELECT name FROM sal_emp WHERE pay_by_quarter[1] <> pay_by_quarter[2];
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name
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-------
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Carol
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(1 row)
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</programlisting>
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The array subscript numbers are written within square brackets.
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<productname>PostgreSQL</productname> uses the
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<quote>one-based</quote> numbering convention for arrays, that is,
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an array of <replaceable>n</> elements starts with <literal>array[1]</literal> and
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ends with <literal>array[<replaceable>n</>]</literal>.
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</para>
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<para>
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This query retrieves the third quarter pay of all employees:
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<programlisting>
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SELECT pay_by_quarter[3] FROM sal_emp;
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pay_by_quarter
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----------------
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10000
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25000
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(2 rows)
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</programlisting>
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</para>
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<para>
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We can also access arbitrary rectangular slices of an array, or
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subarrays. An array slice is denoted by writing
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<literal><replaceable>lower subscript</replaceable> :
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<replaceable>upper subscript</replaceable></literal> for one or more
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array dimensions. This query retrieves the first item on Bill's
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schedule for the first two days of the week:
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<programlisting>
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SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';
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schedule
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--------------------
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{{"meeting"},{""}}
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(1 row)
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</programlisting>
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We could also have written
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<programlisting>
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SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';
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</programlisting>
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with the same result. An array subscripting operation is taken to
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represent an array slice if any of the subscripts are written in the
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form <replaceable>lower</replaceable> <literal>:</literal>
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<replaceable>upper</replaceable>. A lower bound of 1 is assumed for
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any subscript where only one value is specified.
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</para>
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<para>
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An array value can be replaced completely:
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<programlisting>
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UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}'
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WHERE name = 'Carol';
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</programlisting>
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or updated at a single element:
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<programlisting>
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UPDATE sal_emp SET pay_by_quarter[4] = 15000
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WHERE name = 'Bill';
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</programListing>
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or updated in a slice:
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<programlisting>
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UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}'
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WHERE name = 'Carol';
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</programlisting>
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</para>
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<para>
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An array can be enlarged by assigning to an element adjacent to
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those already present, or by assigning to a slice that is adjacent
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to or overlaps the data already present. For example, if an array
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value currently has 4 elements, it will have five elements after an
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update that assigns to array[5]. Currently, enlargement in this
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fashion is only allowed for one-dimensional arrays, not
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multidimensional arrays.
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</para>
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<para>
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The syntax for <command>CREATE TABLE</command> allows fixed-length
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arrays to be defined:
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<programlisting>
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CREATE TABLE tictactoe (
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squares integer[3][3]
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);
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</programlisting>
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However, the current implementation does not enforce the array size
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limits --- the behavior is the same as for arrays of unspecified
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length.
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</para>
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<para>
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Actually, the current implementation does not enforce the declared
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number of dimensions either. Arrays of a particular element type are
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all considered to be of the same type, regardless of size or number
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of dimensions.
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</para>
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<para>
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The current dimensions of any array value can be retrieved with the
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<function>array_dims</function> function:
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<programlisting>
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SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol';
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array_dims
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------------
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[1:2][1:1]
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(1 row)
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</programlisting>
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<function>array_dims</function> produces a <type>text</type> result,
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which is convenient for people to read but perhaps not so convenient
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for programs.
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</para>
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<para>
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To search for a value in an array, you must check each value of the
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array. This can be done by hand (if you know the size of the array):
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<programlisting>
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SELECT * FROM sal_emp WHERE pay_by_quarter[1] = 10000 OR
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pay_by_quarter[2] = 10000 OR
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pay_by_quarter[3] = 10000 OR
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pay_by_quarter[4] = 10000;
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</programlisting>
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However, this quickly becomes tedious for large arrays, and is not
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helpful if the size of the array is unknown. Although it is not part
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of the primary <productname>PostgreSQL</productname> distribution,
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there is an extension available that defines new functions and
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operators for iterating over array values. Using this, the above
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query could be:
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<programlisting>
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SELECT * FROM sal_emp WHERE pay_by_quarter[1:4] *= 10000;
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</programlisting>
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To search the entire array (not just specified columns), you could
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use:
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<programlisting>
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SELECT * FROM sal_emp WHERE pay_by_quarter *= 10000;
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</programlisting>
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In addition, you could find rows where the array had all values
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equal to 10 000 with:
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<programlisting>
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SELECT * FROM sal_emp WHERE pay_by_quarter **= 10000;
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</programlisting>
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To install this optional module, look in the
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<filename>contrib/array</filename> directory of the
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<productname>PostgreSQL</productname> source distribution.
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</para>
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<tip>
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<para>
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Arrays are not sets; using arrays in the manner described in the
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previous paragraph is often a sign of database misdesign. The
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array field should generally be split off into a separate table.
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Tables can obviously be searched easily.
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</para>
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</tip>
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<note>
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<para>
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A limitation of the present array implementation is that individual
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elements of an array cannot be SQL NULLs. The entire array can be set
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to NULL, but you can't have an array with some elements NULL and some
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not. Fixing this is on the TODO list.
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</para>
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</note>
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<formalpara>
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<title>Quoting array elements.</title>
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<para>
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As shown above, when writing an array literal value you may write double
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quotes around any individual array
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element. You <emphasis>must</> do so if the element value would otherwise
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confuse the array-value parser. For example, elements containing curly
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braces, commas, double quotes, backslashes, or white space must be
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double-quoted. To put a double quote or backslash in an array element
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value, precede it with a backslash.
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</para>
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</formalpara>
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<tip>
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<para>
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Remember that what you write in an SQL query will first be interpreted
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as a string literal, and then as an array. This doubles the number of
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backslashes you need. For example, to insert a <type>text</> array
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value containing a backslash and a double quote, you'd need to write
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<programlisting>
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INSERT ... VALUES ('{"\\\\","\\""}');
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</programlisting>
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The string-literal processor removes one level of backslashes, so that
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what arrives at the array-value parser looks like <literal>{"\\","\""}</>.
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In turn, the strings fed to the <type>text</> datatype's input routine
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become <literal>\</> and <literal>"</> respectively. (If we were working
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with a datatype whose input routine also treated backslashes specially,
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<type>bytea</> for example, we might need as many as eight backslashes
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in the query to get one backslash into the stored array element.)
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</para>
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</tip>
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</chapter>
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