1 <!-- $PostgreSQL: pgsql/doc/src/sgml/array.sgml,v 1.67 2008/10/29 11:24:52 petere Exp $ -->
7 <primary>array</primary>
11 <productname>PostgreSQL</productname> allows columns of a table to be
12 defined as variable-length multidimensional arrays. Arrays of any
13 built-in or user-defined base type, enum type, or composite type
15 Arrays of domains are not yet supported.
18 <sect2 id="arrays-declaration">
19 <title>Declaration of Array Types</title>
22 <primary>array</primary>
23 <secondary>declaration</secondary>
27 To illustrate the use of array types, we create this table:
29 CREATE TABLE sal_emp (
31 pay_by_quarter integer[],
35 As shown, an array data type is named by appending square brackets
36 (<literal>[]</>) to the data type name of the array elements. The
37 above command will create a table named
38 <structname>sal_emp</structname> with a column of type
39 <type>text</type> (<structfield>name</structfield>), a
40 one-dimensional array of type <type>integer</type>
41 (<structfield>pay_by_quarter</structfield>), which represents the
42 employee's salary by quarter, and a two-dimensional array of
43 <type>text</type> (<structfield>schedule</structfield>), which
44 represents the employee's weekly schedule.
48 The syntax for <command>CREATE TABLE</command> allows the exact size of
49 arrays to be specified, for example:
52 CREATE TABLE tictactoe (
57 However, the current implementation does not enforce the array size
58 limits — the behavior is the same as for arrays of unspecified
63 Actually, the current implementation does not enforce the declared
64 number of dimensions either. Arrays of a particular element type are
65 all considered to be of the same type, regardless of size or number
66 of dimensions. So, declaring number of dimensions or sizes in
67 <command>CREATE TABLE</command> is simply documentation, it does not
68 affect run-time behavior.
72 An alternative syntax, which conforms to the SQL standard, can
73 be used for one-dimensional arrays.
74 <structfield>pay_by_quarter</structfield> could have been defined
77 pay_by_quarter integer ARRAY[4],
79 Or, if no array size is to be specified:
81 pay_by_quarter integer ARRAY,
83 As before, however, <productname>PostgreSQL</> does not enforce the
84 size restriction in any case.
88 <sect2 id="arrays-input">
89 <title>Array Value Input</title>
92 <primary>array</primary>
93 <secondary>constant</secondary>
97 To write an array value as a literal constant, enclose the element
98 values within curly braces and separate them by commas. (If you
99 know C, this is not unlike the C syntax for initializing
100 structures.) You can put double quotes around any element value,
101 and must do so if it contains commas or curly braces. (More
102 details appear below.) Thus, the general format of an array
103 constant is the following:
105 '{ <replaceable>val1</replaceable> <replaceable>delim</replaceable> <replaceable>val2</replaceable> <replaceable>delim</replaceable> ... }'
107 where <replaceable>delim</replaceable> is the delimiter character
108 for the type, as recorded in its <literal>pg_type</literal> entry.
109 Among the standard data types provided in the
110 <productname>PostgreSQL</productname> distribution, type
111 <literal>box</> uses a semicolon (<literal>;</>) but all the others
112 use comma (<literal>,</>). Each <replaceable>val</replaceable> is
113 either a constant of the array element type, or a subarray. An example
114 of an array constant is:
116 '{{1,2,3},{4,5,6},{7,8,9}}'
118 This constant is a two-dimensional, 3-by-3 array consisting of
119 three subarrays of integers.
123 To set an element of an array constant to NULL, write <literal>NULL</>
124 for the element value. (Any upper- or lower-case variant of
125 <literal>NULL</> will do.) If you want an actual string value
126 <quote>NULL</>, you must put double quotes around it.
130 (These kinds of array constants are actually only a special case of
131 the generic type constants discussed in <xref
132 linkend="sql-syntax-constants-generic">. The constant is initially
133 treated as a string and passed to the array input conversion
134 routine. An explicit type specification might be necessary.)
138 Now we can show some <command>INSERT</command> statements:
143 '{10000, 10000, 10000, 10000}',
144 '{{"meeting", "lunch"}, {"training", "presentation"}}');
148 '{20000, 25000, 25000, 25000}',
149 '{{"breakfast", "consulting"}, {"meeting", "lunch"}}');
154 The result of the previous two inserts looks like this:
157 SELECT * FROM sal_emp;
158 name | pay_by_quarter | schedule
159 -------+---------------------------+-------------------------------------------
160 Bill | {10000,10000,10000,10000} | {{meeting,lunch},{training,presentation}}
161 Carol | {20000,25000,25000,25000} | {{breakfast,consulting},{meeting,lunch}}
167 The <literal>ARRAY</> constructor syntax can also be used:
171 ARRAY[10000, 10000, 10000, 10000],
172 ARRAY[['meeting', 'lunch'], ['training', 'presentation']]);
176 ARRAY[20000, 25000, 25000, 25000],
177 ARRAY[['breakfast', 'consulting'], ['meeting', 'lunch']]);
179 Notice that the array elements are ordinary SQL constants or
180 expressions; for instance, string literals are single quoted, instead of
181 double quoted as they would be in an array literal. The <literal>ARRAY</>
182 constructor syntax is discussed in more detail in
183 <xref linkend="sql-syntax-array-constructors">.
187 Multidimensional arrays must have matching extents for each
188 dimension. A mismatch causes an error report, for example:
193 '{10000, 10000, 10000, 10000}',
194 '{{"meeting", "lunch"}, {"meeting"}}');
195 ERROR: multidimensional arrays must have array expressions with matching dimensions
200 <sect2 id="arrays-accessing">
201 <title>Accessing Arrays</title>
204 <primary>array</primary>
205 <secondary>accessing</secondary>
209 Now, we can run some queries on the table.
210 First, we show how to access a single element of an array at a time.
211 This query retrieves the names of the employees whose pay changed in
215 SELECT name FROM sal_emp WHERE pay_by_quarter[1] <> pay_by_quarter[2];
223 The array subscript numbers are written within square brackets.
224 By default <productname>PostgreSQL</productname> uses the
225 one-based numbering convention for arrays, that is,
226 an array of <replaceable>n</> elements starts with <literal>array[1]</literal> and
227 ends with <literal>array[<replaceable>n</>]</literal>.
231 This query retrieves the third quarter pay of all employees:
234 SELECT pay_by_quarter[3] FROM sal_emp;
245 We can also access arbitrary rectangular slices of an array, or
246 subarrays. An array slice is denoted by writing
247 <literal><replaceable>lower-bound</replaceable>:<replaceable>upper-bound</replaceable></literal>
248 for one or more array dimensions. For example, this query retrieves the first
249 item on Bill's schedule for the first two days of the week:
252 SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';
255 ------------------------
256 {{meeting},{training}}
260 If any dimension is written as a slice, i.e. contains a colon, then all
261 dimensions are treated as slices. Any dimension that has only a single
262 number (no colon) is treated as being from <literal>1</>
263 to the number specified. For example, <literal>[2]</> is treated as
264 <literal>[1:2]</>, as in this example:
267 SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill';
270 -------------------------------------------
271 {{meeting,lunch},{training,presentation}}
275 To avoid confusion with the non-slice case, it's best to use slice syntax
276 for all dimensions, e.g., <literal>[1:2][1:1]</>, not <literal>[2][1:1]</>.
280 An array subscript expression will return null if either the array itself or
281 any of the subscript expressions are null. Also, null is returned if a
282 subscript is outside the array bounds (this case does not raise an error).
283 For example, if <literal>schedule</>
284 currently has the dimensions <literal>[1:3][1:2]</> then referencing
285 <literal>schedule[3][3]</> yields NULL. Similarly, an array reference
286 with the wrong number of subscripts yields a null rather than an error.
290 An array slice expression likewise yields null if the array itself or
291 any of the subscript expressions are null. However, in other corner
292 cases such as selecting an array slice that
293 is completely outside the current array bounds, a slice expression
294 yields an empty (zero-dimensional) array instead of null. (This
295 does not match non-slice behavior and is done for historical reasons.)
296 If the requested slice partially overlaps the array bounds, then it
297 is silently reduced to just the overlapping region.
301 The current dimensions of any array value can be retrieved with the
302 <function>array_dims</function> function:
305 SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol';
313 <function>array_dims</function> produces a <type>text</type> result,
314 which is convenient for people to read but perhaps not so convenient
315 for programs. Dimensions can also be retrieved with
316 <function>array_upper</function> and <function>array_lower</function>,
317 which return the upper and lower bound of a
318 specified array dimension, respectively:
321 SELECT array_upper(schedule, 1) FROM sal_emp WHERE name = 'Carol';
331 <sect2 id="arrays-modifying">
332 <title>Modifying Arrays</title>
335 <primary>array</primary>
336 <secondary>modifying</secondary>
340 An array value can be replaced completely:
343 UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}'
344 WHERE name = 'Carol';
347 or using the <literal>ARRAY</literal> expression syntax:
350 UPDATE sal_emp SET pay_by_quarter = ARRAY[25000,25000,27000,27000]
351 WHERE name = 'Carol';
354 An array can also be updated at a single element:
357 UPDATE sal_emp SET pay_by_quarter[4] = 15000
361 or updated in a slice:
364 UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}'
365 WHERE name = 'Carol';
371 A stored array value can be enlarged by assigning to element(s) not already
372 present. Any positions between those previously present and the newly
373 assigned element(s) will be filled with nulls. For example, if array
374 <literal>myarray</> currently has 4 elements, it will have six
375 elements after an update that assigns to <literal>myarray[6]</>,
376 and <literal>myarray[5]</> will contain a null.
377 Currently, enlargement in this fashion is only allowed for one-dimensional
378 arrays, not multidimensional arrays.
382 Subscripted assignment allows creation of arrays that do not use one-based
383 subscripts. For example one might assign to <literal>myarray[-2:7]</> to
384 create an array with subscript values running from -2 to 7.
388 New array values can also be constructed by using the concatenation operator,
389 <literal>||</literal>:
391 SELECT ARRAY[1,2] || ARRAY[3,4];
397 SELECT ARRAY[5,6] || ARRAY[[1,2],[3,4]];
399 ---------------------
406 The concatenation operator allows a single element to be pushed on to the
407 beginning or end of a one-dimensional array. It also accepts two
408 <replaceable>N</>-dimensional arrays, or an <replaceable>N</>-dimensional
409 and an <replaceable>N+1</>-dimensional array.
413 When a single element is pushed on to either the beginning or end of a
414 one-dimensional array, the result is an array with the same lower bound
415 subscript as the array operand. For example:
417 SELECT array_dims(1 || '[0:1]={2,3}'::int[]);
423 SELECT array_dims(ARRAY[1,2] || 3);
432 When two arrays with an equal number of dimensions are concatenated, the
433 result retains the lower bound subscript of the left-hand operand's outer
434 dimension. The result is an array comprising every element of the left-hand
435 operand followed by every element of the right-hand operand. For example:
437 SELECT array_dims(ARRAY[1,2] || ARRAY[3,4,5]);
443 SELECT array_dims(ARRAY[[1,2],[3,4]] || ARRAY[[5,6],[7,8],[9,0]]);
452 When an <replaceable>N</>-dimensional array is pushed on to the beginning
453 or end of an <replaceable>N+1</>-dimensional array, the result is
454 analogous to the element-array case above. Each <replaceable>N</>-dimensional
455 sub-array is essentially an element of the <replaceable>N+1</>-dimensional
456 array's outer dimension. For example:
458 SELECT array_dims(ARRAY[1,2] || ARRAY[[3,4],[5,6]]);
467 An array can also be constructed by using the functions
468 <function>array_prepend</function>, <function>array_append</function>,
469 or <function>array_cat</function>. The first two only support one-dimensional
470 arrays, but <function>array_cat</function> supports multidimensional arrays.
472 Note that the concatenation operator discussed above is preferred over
473 direct use of these functions. In fact, the functions exist primarily for use
474 in implementing the concatenation operator. However, they might be directly
475 useful in the creation of user-defined aggregates. Some examples:
478 SELECT array_prepend(1, ARRAY[2,3]);
484 SELECT array_append(ARRAY[1,2], 3);
490 SELECT array_cat(ARRAY[1,2], ARRAY[3,4]);
496 SELECT array_cat(ARRAY[[1,2],[3,4]], ARRAY[5,6]);
498 ---------------------
502 SELECT array_cat(ARRAY[5,6], ARRAY[[1,2],[3,4]]);
504 ---------------------
510 <sect2 id="arrays-searching">
511 <title>Searching in Arrays</title>
514 <primary>array</primary>
515 <secondary>searching</secondary>
519 To search for a value in an array, you must check each value of the
520 array. This can be done by hand, if you know the size of the array.
524 SELECT * FROM sal_emp WHERE pay_by_quarter[1] = 10000 OR
525 pay_by_quarter[2] = 10000 OR
526 pay_by_quarter[3] = 10000 OR
527 pay_by_quarter[4] = 10000;
530 However, this quickly becomes tedious for large arrays, and is not
531 helpful if the size of the array is uncertain. An alternative method is
532 described in <xref linkend="functions-comparisons">. The above
533 query could be replaced by:
536 SELECT * FROM sal_emp WHERE 10000 = ANY (pay_by_quarter);
539 In addition, you could find rows where the array had all values
543 SELECT * FROM sal_emp WHERE 10000 = ALL (pay_by_quarter);
549 Alternatively, the <function>generate_subscripts</> function can be used.
554 (SELECT pay_by_quarter,
555 generate_subscripts(pay_by_quarter, 1) AS s
557 WHERE pay_by_quarter[s] = 10000;
560 This function is described in <xref linkend="functions-srf-subscripts">.
565 Arrays are not sets; searching for specific array elements
566 can be a sign of database misdesign. Consider
567 using a separate table with a row for each item that would be an
568 array element. This will be easier to search, and is likely to
569 scale up better to large numbers of elements.
574 <sect2 id="arrays-io">
575 <title>Array Input and Output Syntax</title>
578 <primary>array</primary>
579 <secondary>I/O</secondary>
583 The external text representation of an array value consists of items that
584 are interpreted according to the I/O conversion rules for the array's
585 element type, plus decoration that indicates the array structure.
586 The decoration consists of curly braces (<literal>{</> and <literal>}</>)
587 around the array value plus delimiter characters between adjacent items.
588 The delimiter character is usually a comma (<literal>,</>) but can be
589 something else: it is determined by the <literal>typdelim</> setting
590 for the array's element type. (Among the standard data types provided
591 in the <productname>PostgreSQL</productname> distribution, type
592 <literal>box</> uses a semicolon (<literal>;</>) but all the others
593 use comma.) In a multidimensional array, each dimension (row, plane,
594 cube, etc.) gets its own level of curly braces, and delimiters
595 must be written between adjacent curly-braced entities of the same level.
599 The array output routine will put double quotes around element values
600 if they are empty strings, contain curly braces, delimiter characters,
601 double quotes, backslashes, or white space, or match the word
602 <literal>NULL</>. Double quotes and backslashes
603 embedded in element values will be backslash-escaped. For numeric
604 data types it is safe to assume that double quotes will never appear, but
605 for textual data types one should be prepared to cope with either presence
606 or absence of quotes.
610 By default, the lower bound index value of an array's dimensions is
611 set to one. To represent arrays with other lower bounds, the array
612 subscript ranges can be specified explicitly before writing the
614 This decoration consists of square brackets (<literal>[]</>)
615 around each array dimension's lower and upper bounds, with
616 a colon (<literal>:</>) delimiter character in between. The
617 array dimension decoration is followed by an equal sign (<literal>=</>).
620 SELECT f1[1][-2][3] AS e1, f1[1][-1][5] AS e2
621 FROM (SELECT '[1:1][-2:-1][3:5]={{{1,2,3},{4,5,6}}}'::int[] AS f1) AS ss;
628 The array output routine will include explicit dimensions in its result
629 only when there are one or more lower bounds different from one.
633 If the value written for an element is <literal>NULL</> (in any case
634 variant), the element is taken to be NULL. The presence of any quotes
635 or backslashes disables this and allows the literal string value
636 <quote>NULL</> to be entered. Also, for backwards compatibility with
637 pre-8.2 versions of <productname>PostgreSQL</>, the <xref
638 linkend="guc-array-nulls"> configuration parameter might be turned
639 <literal>off</> to suppress recognition of <literal>NULL</> as a NULL.
643 As shown previously, when writing an array value you can write double
644 quotes around any individual array element. You <emphasis>must</> do so
645 if the element value would otherwise confuse the array-value parser.
646 For example, elements containing curly braces, commas (or whatever the
647 delimiter character is), double quotes, backslashes, or leading or trailing
648 whitespace must be double-quoted. Empty strings and strings matching the
649 word <literal>NULL</> must be quoted, too. To put a double quote or
650 backslash in a quoted array element value, use escape string syntax
651 and precede it with a backslash. Alternatively, you can use
652 backslash-escaping to protect all data characters that would otherwise
653 be taken as array syntax.
657 You can write whitespace before a left brace or after a right
658 brace. You can also write whitespace before or after any individual item
659 string. In all of these cases the whitespace will be ignored. However,
660 whitespace within double-quoted elements, or surrounded on both sides by
661 non-whitespace characters of an element, is not ignored.
666 Remember that what you write in an SQL command will first be interpreted
667 as a string literal, and then as an array. This doubles the number of
668 backslashes you need. For example, to insert a <type>text</> array
669 value containing a backslash and a double quote, you'd need to write:
671 INSERT ... VALUES (E'{"\\\\","\\""}');
673 The escape string processor removes one level of backslashes, so that
674 what arrives at the array-value parser looks like <literal>{"\\","\""}</>.
675 In turn, the strings fed to the <type>text</> data type's input routine
676 become <literal>\</> and <literal>"</> respectively. (If we were working
677 with a data type whose input routine also treated backslashes specially,
678 <type>bytea</> for example, we might need as many as eight backslashes
679 in the command to get one backslash into the stored array element.)
680 Dollar quoting (see <xref linkend="sql-syntax-dollar-quoting">) can be
681 used to avoid the need to double backslashes.
687 The <literal>ARRAY</> constructor syntax (see
688 <xref linkend="sql-syntax-array-constructors">) is often easier to work
689 with than the array-literal syntax when writing array values in SQL
690 commands. In <literal>ARRAY</>, individual element values are written the
691 same way they would be written when not members of an array.