diff options
Diffstat (limited to '')
| -rw-r--r-- | doc/src/sgml/html/arrays.html | 647 |
1 files changed, 647 insertions, 0 deletions
diff --git a/doc/src/sgml/html/arrays.html b/doc/src/sgml/html/arrays.html new file mode 100644 index 0000000..c7b097e --- /dev/null +++ b/doc/src/sgml/html/arrays.html @@ -0,0 +1,647 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>8.15. Arrays</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="datatype-json.html" title="8.14. JSON Types" /><link rel="next" href="rowtypes.html" title="8.16. Composite Types" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">8.15. Arrays</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="datatype-json.html" title="8.14. JSON Types">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="datatype.html" title="Chapter 8. Data Types">Up</a></td><th width="60%" align="center">Chapter 8. Data Types</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 14.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="rowtypes.html" title="8.16. Composite Types">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="ARRAYS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">8.15. Arrays</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="arrays.html#ARRAYS-DECLARATION">8.15.1. Declaration of Array Types</a></span></dt><dt><span class="sect2"><a href="arrays.html#ARRAYS-INPUT">8.15.2. Array Value Input</a></span></dt><dt><span class="sect2"><a href="arrays.html#ARRAYS-ACCESSING">8.15.3. Accessing Arrays</a></span></dt><dt><span class="sect2"><a href="arrays.html#ARRAYS-MODIFYING">8.15.4. Modifying Arrays</a></span></dt><dt><span class="sect2"><a href="arrays.html#ARRAYS-SEARCHING">8.15.5. Searching in Arrays</a></span></dt><dt><span class="sect2"><a href="arrays.html#ARRAYS-IO">8.15.6. Array Input and Output Syntax</a></span></dt></dl></div><a id="id-1.5.7.23.2" class="indexterm"></a><p> + <span class="productname">PostgreSQL</span> allows columns of a table to be + defined as variable-length multidimensional arrays. Arrays of any + built-in or user-defined base type, enum type, composite type, range type, + or domain can be created. + </p><div class="sect2" id="ARRAYS-DECLARATION"><div class="titlepage"><div><div><h3 class="title">8.15.1. Declaration of Array Types</h3></div></div></div><a id="id-1.5.7.23.4.2" class="indexterm"></a><p> + To illustrate the use of array types, we create this table: +</p><pre class="programlisting"> +CREATE TABLE sal_emp ( + name text, + pay_by_quarter integer[], + schedule text[][] +); +</pre><p> + As shown, an array data type is named by appending square brackets + (<code class="literal">[]</code>) to the data type name of the array elements. The + above command will create a table named + <code class="structname">sal_emp</code> with a column of type + <code class="type">text</code> (<code class="structfield">name</code>), a + one-dimensional array of type <code class="type">integer</code> + (<code class="structfield">pay_by_quarter</code>), which represents the + employee's salary by quarter, and a two-dimensional array of + <code class="type">text</code> (<code class="structfield">schedule</code>), which + represents the employee's weekly schedule. + </p><p> + The syntax for <code class="command">CREATE TABLE</code> allows the exact size of + arrays to be specified, for example: + +</p><pre class="programlisting"> +CREATE TABLE tictactoe ( + squares integer[3][3] +); +</pre><p> + + However, the current implementation ignores any supplied array size + limits, i.e., the behavior is the same as for arrays of unspecified + length. + </p><p> + The current implementation does not enforce the declared + number of dimensions either. Arrays of a particular element type are + all considered to be of the same type, regardless of size or number + of dimensions. So, declaring the array size or number of dimensions in + <code class="command">CREATE TABLE</code> is simply documentation; it does not + affect run-time behavior. + </p><p> + An alternative syntax, which conforms to the SQL standard by using + the keyword <code class="literal">ARRAY</code>, can be used for one-dimensional arrays. + <code class="structfield">pay_by_quarter</code> could have been defined + as: +</p><pre class="programlisting"> + pay_by_quarter integer ARRAY[4], +</pre><p> + Or, if no array size is to be specified: +</p><pre class="programlisting"> + pay_by_quarter integer ARRAY, +</pre><p> + As before, however, <span class="productname">PostgreSQL</span> does not enforce the + size restriction in any case. + </p></div><div class="sect2" id="ARRAYS-INPUT"><div class="titlepage"><div><div><h3 class="title">8.15.2. Array Value Input</h3></div></div></div><a id="id-1.5.7.23.5.2" class="indexterm"></a><p> + To write an array value as a literal constant, enclose the element + values within curly braces and separate them by commas. (If you + know C, this is not unlike the C syntax for initializing + structures.) You can put double quotes around any element value, + and must do so if it contains commas or curly braces. (More + details appear below.) Thus, the general format of an array + constant is the following: +</p><pre class="synopsis"> +'{ <em class="replaceable"><code>val1</code></em> <em class="replaceable"><code>delim</code></em> <em class="replaceable"><code>val2</code></em> <em class="replaceable"><code>delim</code></em> ... }' +</pre><p> + where <em class="replaceable"><code>delim</code></em> is the delimiter character + for the type, as recorded in its <code class="literal">pg_type</code> entry. + Among the standard data types provided in the + <span class="productname">PostgreSQL</span> distribution, all use a comma + (<code class="literal">,</code>), except for type <code class="type">box</code> which uses a semicolon + (<code class="literal">;</code>). Each <em class="replaceable"><code>val</code></em> is + either a constant of the array element type, or a subarray. An example + of an array constant is: +</p><pre class="programlisting"> +'{{1,2,3},{4,5,6},{7,8,9}}' +</pre><p> + This constant is a two-dimensional, 3-by-3 array consisting of + three subarrays of integers. + </p><p> + To set an element of an array constant to NULL, write <code class="literal">NULL</code> + for the element value. (Any upper- or lower-case variant of + <code class="literal">NULL</code> will do.) If you want an actual string value + <span class="quote">“<span class="quote">NULL</span>”</span>, you must put double quotes around it. + </p><p> + (These kinds of array constants are actually only a special case of + the generic type constants discussed in <a class="xref" href="sql-syntax-lexical.html#SQL-SYNTAX-CONSTANTS-GENERIC" title="4.1.2.7. Constants of Other Types">Section 4.1.2.7</a>. The constant is initially + treated as a string and passed to the array input conversion + routine. An explicit type specification might be necessary.) + </p><p> + Now we can show some <code class="command">INSERT</code> statements: + +</p><pre class="programlisting"> +INSERT INTO sal_emp + VALUES ('Bill', + '{10000, 10000, 10000, 10000}', + '{{"meeting", "lunch"}, {"training", "presentation"}}'); + +INSERT INTO sal_emp + VALUES ('Carol', + '{20000, 25000, 25000, 25000}', + '{{"breakfast", "consulting"}, {"meeting", "lunch"}}'); +</pre><p> + </p><p> + The result of the previous two inserts looks like this: + +</p><pre class="programlisting"> +SELECT * FROM sal_emp; + name | pay_by_quarter | schedule +-------+---------------------------+------------------------------------------- + Bill | {10000,10000,10000,10000} | {{meeting,lunch},{training,presentation}} + Carol | {20000,25000,25000,25000} | {{breakfast,consulting},{meeting,lunch}} +(2 rows) +</pre><p> + </p><p> + Multidimensional arrays must have matching extents for each + dimension. A mismatch causes an error, for example: + +</p><pre class="programlisting"> +INSERT INTO sal_emp + VALUES ('Bill', + '{10000, 10000, 10000, 10000}', + '{{"meeting", "lunch"}, {"meeting"}}'); +ERROR: multidimensional arrays must have array expressions with matching dimensions +</pre><p> + </p><p> + The <code class="literal">ARRAY</code> constructor syntax can also be used: +</p><pre class="programlisting"> +INSERT INTO sal_emp + VALUES ('Bill', + ARRAY[10000, 10000, 10000, 10000], + ARRAY[['meeting', 'lunch'], ['training', 'presentation']]); + +INSERT INTO sal_emp + VALUES ('Carol', + ARRAY[20000, 25000, 25000, 25000], + ARRAY[['breakfast', 'consulting'], ['meeting', 'lunch']]); +</pre><p> + Notice that the array elements are ordinary SQL constants or + expressions; for instance, string literals are single quoted, instead of + double quoted as they would be in an array literal. The <code class="literal">ARRAY</code> + constructor syntax is discussed in more detail in + <a class="xref" href="sql-expressions.html#SQL-SYNTAX-ARRAY-CONSTRUCTORS" title="4.2.12. Array Constructors">Section 4.2.12</a>. + </p></div><div class="sect2" id="ARRAYS-ACCESSING"><div class="titlepage"><div><div><h3 class="title">8.15.3. Accessing Arrays</h3></div></div></div><a id="id-1.5.7.23.6.2" class="indexterm"></a><p> + Now, we can run some queries on the table. + First, we show how to access a single element of an array. + This query retrieves the names of the employees whose pay changed in + the second quarter: + +</p><pre class="programlisting"> +SELECT name FROM sal_emp WHERE pay_by_quarter[1] <> pay_by_quarter[2]; + + name +------- + Carol +(1 row) +</pre><p> + + The array subscript numbers are written within square brackets. + By default <span class="productname">PostgreSQL</span> uses a + one-based numbering convention for arrays, that is, + an array of <em class="replaceable"><code>n</code></em> elements starts with <code class="literal">array[1]</code> and + ends with <code class="literal">array[<em class="replaceable"><code>n</code></em>]</code>. + </p><p> + This query retrieves the third quarter pay of all employees: + +</p><pre class="programlisting"> +SELECT pay_by_quarter[3] FROM sal_emp; + + pay_by_quarter +---------------- + 10000 + 25000 +(2 rows) +</pre><p> + </p><p> + We can also access arbitrary rectangular slices of an array, or + subarrays. An array slice is denoted by writing + <code class="literal"><em class="replaceable"><code>lower-bound</code></em>:<em class="replaceable"><code>upper-bound</code></em></code> + for one or more array dimensions. For example, this query retrieves the first + item on Bill's schedule for the first two days of the week: + +</p><pre class="programlisting"> +SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill'; + + schedule +------------------------ + {{meeting},{training}} +(1 row) +</pre><p> + + If any dimension is written as a slice, i.e., contains a colon, then all + dimensions are treated as slices. Any dimension that has only a single + number (no colon) is treated as being from 1 + to the number specified. For example, <code class="literal">[2]</code> is treated as + <code class="literal">[1:2]</code>, as in this example: + +</p><pre class="programlisting"> +SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill'; + + schedule +------------------------------------------- + {{meeting,lunch},{training,presentation}} +(1 row) +</pre><p> + + To avoid confusion with the non-slice case, it's best to use slice syntax + for all dimensions, e.g., <code class="literal">[1:2][1:1]</code>, not <code class="literal">[2][1:1]</code>. + </p><p> + It is possible to omit the <em class="replaceable"><code>lower-bound</code></em> and/or + <em class="replaceable"><code>upper-bound</code></em> of a slice specifier; the missing + bound is replaced by the lower or upper limit of the array's subscripts. + For example: + +</p><pre class="programlisting"> +SELECT schedule[:2][2:] FROM sal_emp WHERE name = 'Bill'; + + schedule +------------------------ + {{lunch},{presentation}} +(1 row) + +SELECT schedule[:][1:1] FROM sal_emp WHERE name = 'Bill'; + + schedule +------------------------ + {{meeting},{training}} +(1 row) +</pre><p> + </p><p> + An array subscript expression will return null if either the array itself or + any of the subscript expressions are null. Also, null is returned if a + subscript is outside the array bounds (this case does not raise an error). + For example, if <code class="literal">schedule</code> + currently has the dimensions <code class="literal">[1:3][1:2]</code> then referencing + <code class="literal">schedule[3][3]</code> yields NULL. Similarly, an array reference + with the wrong number of subscripts yields a null rather than an error. + </p><p> + An array slice expression likewise yields null if the array itself or + any of the subscript expressions are null. However, in other + cases such as selecting an array slice that + is completely outside the current array bounds, a slice expression + yields an empty (zero-dimensional) array instead of null. (This + does not match non-slice behavior and is done for historical reasons.) + If the requested slice partially overlaps the array bounds, then it + is silently reduced to just the overlapping region instead of + returning null. + </p><p> + The current dimensions of any array value can be retrieved with the + <code class="function">array_dims</code> function: + +</p><pre class="programlisting"> +SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol'; + + array_dims +------------ + [1:2][1:2] +(1 row) +</pre><p> + + <code class="function">array_dims</code> produces a <code class="type">text</code> result, + which is convenient for people to read but perhaps inconvenient + for programs. Dimensions can also be retrieved with + <code class="function">array_upper</code> and <code class="function">array_lower</code>, + which return the upper and lower bound of a + specified array dimension, respectively: + +</p><pre class="programlisting"> +SELECT array_upper(schedule, 1) FROM sal_emp WHERE name = 'Carol'; + + array_upper +------------- + 2 +(1 row) +</pre><p> + + <code class="function">array_length</code> will return the length of a specified + array dimension: + +</p><pre class="programlisting"> +SELECT array_length(schedule, 1) FROM sal_emp WHERE name = 'Carol'; + + array_length +-------------- + 2 +(1 row) +</pre><p> + + <code class="function">cardinality</code> returns the total number of elements in an + array across all dimensions. It is effectively the number of rows a call to + <code class="function">unnest</code> would yield: + +</p><pre class="programlisting"> +SELECT cardinality(schedule) FROM sal_emp WHERE name = 'Carol'; + + cardinality +------------- + 4 +(1 row) +</pre><p> + </p></div><div class="sect2" id="ARRAYS-MODIFYING"><div class="titlepage"><div><div><h3 class="title">8.15.4. Modifying Arrays</h3></div></div></div><a id="id-1.5.7.23.7.2" class="indexterm"></a><p> + An array value can be replaced completely: + +</p><pre class="programlisting"> +UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}' + WHERE name = 'Carol'; +</pre><p> + + or using the <code class="literal">ARRAY</code> expression syntax: + +</p><pre class="programlisting"> +UPDATE sal_emp SET pay_by_quarter = ARRAY[25000,25000,27000,27000] + WHERE name = 'Carol'; +</pre><p> + + An array can also be updated at a single element: + +</p><pre class="programlisting"> +UPDATE sal_emp SET pay_by_quarter[4] = 15000 + WHERE name = 'Bill'; +</pre><p> + + or updated in a slice: + +</p><pre class="programlisting"> +UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}' + WHERE name = 'Carol'; +</pre><p> + + The slice syntaxes with omitted <em class="replaceable"><code>lower-bound</code></em> and/or + <em class="replaceable"><code>upper-bound</code></em> can be used too, but only when + updating an array value that is not NULL or zero-dimensional (otherwise, + there is no existing subscript limit to substitute). + </p><p> + A stored array value can be enlarged by assigning to elements not already + present. Any positions between those previously present and the newly + assigned elements will be filled with nulls. For example, if array + <code class="literal">myarray</code> currently has 4 elements, it will have six + elements after an update that assigns to <code class="literal">myarray[6]</code>; + <code class="literal">myarray[5]</code> will contain null. + Currently, enlargement in this fashion is only allowed for one-dimensional + arrays, not multidimensional arrays. + </p><p> + Subscripted assignment allows creation of arrays that do not use one-based + subscripts. For example one might assign to <code class="literal">myarray[-2:7]</code> to + create an array with subscript values from -2 to 7. + </p><p> + New array values can also be constructed using the concatenation operator, + <code class="literal">||</code>: +</p><pre class="programlisting"> +SELECT ARRAY[1,2] || ARRAY[3,4]; + ?column? +----------- + {1,2,3,4} +(1 row) + +SELECT ARRAY[5,6] || ARRAY[[1,2],[3,4]]; + ?column? +--------------------- + {{5,6},{1,2},{3,4}} +(1 row) +</pre><p> + </p><p> + The concatenation operator allows a single element to be pushed onto the + beginning or end of a one-dimensional array. It also accepts two + <em class="replaceable"><code>N</code></em>-dimensional arrays, or an <em class="replaceable"><code>N</code></em>-dimensional + and an <em class="replaceable"><code>N+1</code></em>-dimensional array. + </p><p> + When a single element is pushed onto either the beginning or end of a + one-dimensional array, the result is an array with the same lower bound + subscript as the array operand. For example: +</p><pre class="programlisting"> +SELECT array_dims(1 || '[0:1]={2,3}'::int[]); + array_dims +------------ + [0:2] +(1 row) + +SELECT array_dims(ARRAY[1,2] || 3); + array_dims +------------ + [1:3] +(1 row) +</pre><p> + </p><p> + When two arrays with an equal number of dimensions are concatenated, the + result retains the lower bound subscript of the left-hand operand's outer + dimension. The result is an array comprising every element of the left-hand + operand followed by every element of the right-hand operand. For example: +</p><pre class="programlisting"> +SELECT array_dims(ARRAY[1,2] || ARRAY[3,4,5]); + array_dims +------------ + [1:5] +(1 row) + +SELECT array_dims(ARRAY[[1,2],[3,4]] || ARRAY[[5,6],[7,8],[9,0]]); + array_dims +------------ + [1:5][1:2] +(1 row) +</pre><p> + </p><p> + When an <em class="replaceable"><code>N</code></em>-dimensional array is pushed onto the beginning + or end of an <em class="replaceable"><code>N+1</code></em>-dimensional array, the result is + analogous to the element-array case above. Each <em class="replaceable"><code>N</code></em>-dimensional + sub-array is essentially an element of the <em class="replaceable"><code>N+1</code></em>-dimensional + array's outer dimension. For example: +</p><pre class="programlisting"> +SELECT array_dims(ARRAY[1,2] || ARRAY[[3,4],[5,6]]); + array_dims +------------ + [1:3][1:2] +(1 row) +</pre><p> + </p><p> + An array can also be constructed by using the functions + <code class="function">array_prepend</code>, <code class="function">array_append</code>, + or <code class="function">array_cat</code>. The first two only support one-dimensional + arrays, but <code class="function">array_cat</code> supports multidimensional arrays. + Some examples: + +</p><pre class="programlisting"> +SELECT array_prepend(1, ARRAY[2,3]); + array_prepend +--------------- + {1,2,3} +(1 row) + +SELECT array_append(ARRAY[1,2], 3); + array_append +-------------- + {1,2,3} +(1 row) + +SELECT array_cat(ARRAY[1,2], ARRAY[3,4]); + array_cat +----------- + {1,2,3,4} +(1 row) + +SELECT array_cat(ARRAY[[1,2],[3,4]], ARRAY[5,6]); + array_cat +--------------------- + {{1,2},{3,4},{5,6}} +(1 row) + +SELECT array_cat(ARRAY[5,6], ARRAY[[1,2],[3,4]]); + array_cat +--------------------- + {{5,6},{1,2},{3,4}} +</pre><p> + </p><p> + In simple cases, the concatenation operator discussed above is preferred + over direct use of these functions. However, because the concatenation + operator is overloaded to serve all three cases, there are situations where + use of one of the functions is helpful to avoid ambiguity. For example + consider: + +</p><pre class="programlisting"> +SELECT ARRAY[1, 2] || '{3, 4}'; -- the untyped literal is taken as an array + ?column? +----------- + {1,2,3,4} + +SELECT ARRAY[1, 2] || '7'; -- so is this one +ERROR: malformed array literal: "7" + +SELECT ARRAY[1, 2] || NULL; -- so is an undecorated NULL + ?column? +---------- + {1,2} +(1 row) + +SELECT array_append(ARRAY[1, 2], NULL); -- this might have been meant + array_append +-------------- + {1,2,NULL} +</pre><p> + + In the examples above, the parser sees an integer array on one side of the + concatenation operator, and a constant of undetermined type on the other. + The heuristic it uses to resolve the constant's type is to assume it's of + the same type as the operator's other input — in this case, + integer array. So the concatenation operator is presumed to + represent <code class="function">array_cat</code>, not <code class="function">array_append</code>. When + that's the wrong choice, it could be fixed by casting the constant to the + array's element type; but explicit use of <code class="function">array_append</code> might + be a preferable solution. + </p></div><div class="sect2" id="ARRAYS-SEARCHING"><div class="titlepage"><div><div><h3 class="title">8.15.5. Searching in Arrays</h3></div></div></div><a id="id-1.5.7.23.8.2" class="indexterm"></a><p> + To search for a value in an array, each value must be checked. + This can be done manually, if you know the size of the array. + For example: + +</p><pre class="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; +</pre><p> + + However, this quickly becomes tedious for large arrays, and is not + helpful if the size of the array is unknown. An alternative method is + described in <a class="xref" href="functions-comparisons.html" title="9.24. Row and Array Comparisons">Section 9.24</a>. The above + query could be replaced by: + +</p><pre class="programlisting"> +SELECT * FROM sal_emp WHERE 10000 = ANY (pay_by_quarter); +</pre><p> + + In addition, you can find rows where the array has all values + equal to 10000 with: + +</p><pre class="programlisting"> +SELECT * FROM sal_emp WHERE 10000 = ALL (pay_by_quarter); +</pre><p> + + </p><p> + Alternatively, the <code class="function">generate_subscripts</code> function can be used. + For example: + +</p><pre class="programlisting"> +SELECT * FROM + (SELECT pay_by_quarter, + generate_subscripts(pay_by_quarter, 1) AS s + FROM sal_emp) AS foo + WHERE pay_by_quarter[s] = 10000; +</pre><p> + + This function is described in <a class="xref" href="functions-srf.html#FUNCTIONS-SRF-SUBSCRIPTS" title="Table 9.64. Subscript Generating Functions">Table 9.64</a>. + </p><p> + You can also search an array using the <code class="literal">&&</code> operator, + which checks whether the left operand overlaps with the right operand. + For instance: + +</p><pre class="programlisting"> +SELECT * FROM sal_emp WHERE pay_by_quarter && ARRAY[10000]; +</pre><p> + + This and other array operators are further described in + <a class="xref" href="functions-array.html" title="9.19. Array Functions and Operators">Section 9.19</a>. It can be accelerated by an appropriate + index, as described in <a class="xref" href="indexes-types.html" title="11.2. Index Types">Section 11.2</a>. + </p><p> + You can also search for specific values in an array using the <code class="function">array_position</code> + and <code class="function">array_positions</code> functions. The former returns the subscript of + the first occurrence of a value in an array; the latter returns an array with the + subscripts of all occurrences of the value in the array. For example: + +</p><pre class="programlisting"> +SELECT array_position(ARRAY['sun','mon','tue','wed','thu','fri','sat'], 'mon'); + array_position +---------------- + 2 +(1 row) + +SELECT array_positions(ARRAY[1, 4, 3, 1, 3, 4, 2, 1], 1); + array_positions +----------------- + {1,4,8} +(1 row) +</pre><p> + </p><div class="tip"><h3 class="title">Tip</h3><p> + Arrays are not sets; searching for specific array elements + can be a sign of database misdesign. Consider + using a separate table with a row for each item that would be an + array element. This will be easier to search, and is likely to + scale better for a large number of elements. + </p></div></div><div class="sect2" id="ARRAYS-IO"><div class="titlepage"><div><div><h3 class="title">8.15.6. Array Input and Output Syntax</h3></div></div></div><a id="id-1.5.7.23.9.2" class="indexterm"></a><p> + The external text representation of an array value consists of items that + are interpreted according to the I/O conversion rules for the array's + element type, plus decoration that indicates the array structure. + The decoration consists of curly braces (<code class="literal">{</code> and <code class="literal">}</code>) + around the array value plus delimiter characters between adjacent items. + The delimiter character is usually a comma (<code class="literal">,</code>) but can be + something else: it is determined by the <code class="literal">typdelim</code> setting + for the array's element type. Among the standard data types provided + in the <span class="productname">PostgreSQL</span> distribution, all use a comma, + except for type <code class="type">box</code>, which uses a semicolon (<code class="literal">;</code>). + In a multidimensional array, each dimension (row, plane, + cube, etc.) gets its own level of curly braces, and delimiters + must be written between adjacent curly-braced entities of the same level. + </p><p> + The array output routine will put double quotes around element values + if they are empty strings, contain curly braces, delimiter characters, + double quotes, backslashes, or white space, or match the word + <code class="literal">NULL</code>. Double quotes and backslashes + embedded in element values will be backslash-escaped. For numeric + data types it is safe to assume that double quotes will never appear, but + for textual data types one should be prepared to cope with either the presence + or absence of quotes. + </p><p> + By default, the lower bound index value of an array's dimensions is + set to one. To represent arrays with other lower bounds, the array + subscript ranges can be specified explicitly before writing the + array contents. + This decoration consists of square brackets (<code class="literal">[]</code>) + around each array dimension's lower and upper bounds, with + a colon (<code class="literal">:</code>) delimiter character in between. The + array dimension decoration is followed by an equal sign (<code class="literal">=</code>). + For example: +</p><pre class="programlisting"> +SELECT f1[1][-2][3] AS e1, f1[1][-1][5] AS e2 + FROM (SELECT '[1:1][-2:-1][3:5]={{{1,2,3},{4,5,6}}}'::int[] AS f1) AS ss; + + e1 | e2 +----+---- + 1 | 6 +(1 row) +</pre><p> + The array output routine will include explicit dimensions in its result + only when there are one or more lower bounds different from one. + </p><p> + If the value written for an element is <code class="literal">NULL</code> (in any case + variant), the element is taken to be NULL. The presence of any quotes + or backslashes disables this and allows the literal string value + <span class="quote">“<span class="quote">NULL</span>”</span> to be entered. Also, for backward compatibility with + pre-8.2 versions of <span class="productname">PostgreSQL</span>, the <a class="xref" href="runtime-config-compatible.html#GUC-ARRAY-NULLS">array_nulls</a> configuration parameter can be turned + <code class="literal">off</code> to suppress recognition of <code class="literal">NULL</code> as a NULL. + </p><p> + As shown previously, when writing an array value you can use double + quotes around any individual array element. You <span class="emphasis"><em>must</em></span> do so + if the element value would otherwise confuse the array-value parser. + For example, elements containing curly braces, commas (or the data type's + delimiter character), double quotes, backslashes, or leading or trailing + whitespace must be double-quoted. Empty strings and strings matching the + word <code class="literal">NULL</code> must be quoted, too. To put a double + quote or backslash in a quoted array element value, precede it + with a backslash. Alternatively, you can avoid quotes and use + backslash-escaping to protect all data characters that would otherwise + be taken as array syntax. + </p><p> + You can add whitespace before a left brace or after a right + brace. You can also add whitespace before or after any individual item + string. In all of these cases the whitespace will be ignored. However, + whitespace within double-quoted elements, or surrounded on both sides by + non-whitespace characters of an element, is not ignored. + </p><div class="tip"><h3 class="title">Tip</h3><p> + The <code class="literal">ARRAY</code> constructor syntax (see + <a class="xref" href="sql-expressions.html#SQL-SYNTAX-ARRAY-CONSTRUCTORS" title="4.2.12. Array Constructors">Section 4.2.12</a>) is often easier to work + with than the array-literal syntax when writing array values in SQL + commands. In <code class="literal">ARRAY</code>, individual element values are written the + same way they would be written when not members of an array. + </p></div></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="datatype-json.html" title="8.14. JSON Types">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="datatype.html" title="Chapter 8. Data Types">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="rowtypes.html" title="8.16. Composite Types">Next</a></td></tr><tr><td width="40%" align="left" valign="top">8.14. <acronym xmlns="http://www.w3.org/1999/xhtml" class="acronym">JSON</acronym> Types </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 14.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 8.16. Composite Types</td></tr></table></div></body></html>
\ No newline at end of file |