PostgreSQL 8.1.23 Documentation | ||||
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PostgreSQL allows columns of a table to be defined as variable-length multidimensional arrays. Arrays of any built-in or user-defined base type can be created. (Arrays of composite types or domains are not yet supported, however.)
To illustrate the use of array types, we create this table:
CREATE TABLE sal_emp ( name text, pay_by_quarter integer[], schedule text[][] );
As shown, an array data type is named by appending square brackets ([]) to the data type name of the array elements. The above command will create a table named sal_emp with a column of type text (name), a one-dimensional array of type integer (pay_by_quarter), which represents the employee's salary by quarter, and a two-dimensional array of text (schedule), which represents the employee's weekly schedule.
The syntax for CREATE TABLE allows the exact size of arrays to be specified, for example:
CREATE TABLE tictactoe ( squares integer[3][3] );
However, the current implementation does not enforce the array size limits — the behavior is the same as for arrays of unspecified length.
Actually, 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 number of dimensions or sizes in CREATE TABLE is simply documentation, it does not affect run-time behavior.
An alternative syntax, which conforms to the SQL standard, may be used for one-dimensional arrays. pay_by_quarter could have been defined as:
pay_by_quarter integer ARRAY[4],
This syntax requires an integer constant to denote the array size. As before, however, PostgreSQL does not enforce the size restriction.
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 may 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:
'{ val1 delim val2 delim ... }'
where delim is the delimiter character for the type, as recorded in its pg_type entry. Among the standard data types provided in the PostgreSQL distribution, type box uses a semicolon (;) but all the others use comma (,). Each val is either a constant of the array element type, or a subarray. An example of an array constant is
'{{1,2,3},{4,5,6},{7,8,9}}'
This constant is a two-dimensional, 3-by-3 array consisting of three subarrays of integers.
(These kinds of array constants are actually only a special case of the generic type constants discussed in Section 4.1.2.5. The constant is initially treated as a string and passed to the array input conversion routine. An explicit type specification might be necessary.)
Now we can show some INSERT statements.
INSERT INTO sal_emp VALUES ('Bill', '{10000, 10000, 10000, 10000}', '{{"meeting", "lunch"}, {"meeting"}}'); ERROR: multidimensional arrays must have array expressions with matching dimensions
Note that multidimensional arrays must have matching extents for each dimension. A mismatch causes an error report.
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"}}');
A limitation of the present array implementation is that individual elements of an array cannot be SQL null values. The entire array can be set to null, but you can't have an array with some elements null and some not. (This is likely to change in the future.)
The result of the previous two inserts looks like this:
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)
The ARRAY constructor syntax may also be used:
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']]);
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 ARRAY constructor syntax is discussed in more detail in Section 4.2.10.
Now, we can run some queries on the table. 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:
SELECT name FROM sal_emp WHERE pay_by_quarter[1] <> pay_by_quarter[2]; name ------- Carol (1 row)
The array subscript numbers are written within square brackets. By default PostgreSQL uses the one-based numbering convention for arrays, that is, an array of n elements starts with array[1] and ends with array[n].
This query retrieves the third quarter pay of all employees:
SELECT pay_by_quarter[3] FROM sal_emp; pay_by_quarter ---------------- 10000 25000 (2 rows)
We can also access arbitrary rectangular slices of an array, or subarrays. An array slice is denoted by writing lower-bound:upper-bound 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:
SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill'; schedule ------------------------ {{meeting},{training}} (1 row)
We could also have written
SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';
with the same result. An array subscripting operation is always taken to represent an array slice if any of the subscripts are written in the form lower:upper. A lower bound of 1 is assumed for any subscript where only one value is specified, as in this example:
SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill'; schedule ------------------------------------------- {{meeting,lunch},{training,presentation}} (1 row)
Fetching from outside the current bounds of an array yields a SQL null value, not an error. For example, if schedule currently has the dimensions [1:3][1:2] then referencing schedule[3][3] yields NULL. Similarly, an array reference with the wrong number of subscripts yields a null rather than an error. Fetching an array slice that is completely outside the current bounds likewise yields a null array; but if the requested slice partially overlaps the array bounds, then it is silently reduced to just the overlapping region.
The current dimensions of any array value can be retrieved
with the array_dims
function:
SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol'; array_dims ------------ [1:2][1:2] (1 row)
array_dims
produces a
text result, which is convenient for
people to read but perhaps not so convenient for programs.
Dimensions can also be retrieved with array_upper
and array_lower
, which return the upper and lower
bound of a specified array dimension, respectively.
SELECT array_upper(schedule, 1) FROM sal_emp WHERE name = 'Carol'; array_upper ------------- 2 (1 row)
An array value can be replaced completely:
UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}' WHERE name = 'Carol';
or using the ARRAY expression syntax:
UPDATE sal_emp SET pay_by_quarter = ARRAY[25000,25000,27000,27000] WHERE name = 'Carol';
An array may also be updated at a single element:
UPDATE sal_emp SET pay_by_quarter[4] = 15000 WHERE name = 'Bill';
or updated in a slice:
UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}' WHERE name = 'Carol';
A stored array value 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 array myarray currently has 4 elements, it will have five elements after an update that assigns to myarray[5]. Currently, enlargement in this fashion is only allowed for one-dimensional arrays, not multidimensional arrays.
Array slice assignment allows creation of arrays that do not use one-based subscripts. For example one might assign to myarray[-2:7] to create an array with subscript values running from -2 to 7.
New array values can also be constructed by using the concatenation operator, ||.
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)
The concatenation operator allows a single element to be pushed on to the beginning or end of a one-dimensional array. It also accepts two N-dimensional arrays, or an N-dimensional and an N+1-dimensional array.
When a single element is pushed on to the beginning of a one-dimensional array, the result is an array with a lower bound subscript equal to the right-hand operand's lower bound subscript, minus one. When a single element is pushed on to the end of a one-dimensional array, the result is an array retaining the lower bound of the left-hand operand. For example:
SELECT array_dims(1 || ARRAY[2,3]); array_dims ------------ [0:2] (1 row) SELECT array_dims(ARRAY[1,2] || 3); array_dims ------------ [1:3] (1 row)
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:
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)
When an N-dimensional array is pushed on to the beginning or end of an N+1-dimensional array, the result is analogous to the element-array case above. Each N-dimensional sub-array is essentially an element of the N+1-dimensional array's outer dimension. For example:
SELECT array_dims(ARRAY[1,2] || ARRAY[[3,4],[5,6]]); array_dims ------------ [0:2][1:2] (1 row)
An array can also be constructed by using the functions
array_prepend
, array_append
, or array_cat
. The first two only support
one-dimensional arrays, but array_cat
supports multidimensional arrays.
Note that the concatenation operator discussed above is
preferred over direct use of these functions. In fact, the
functions are primarily for use in implementing the
concatenation operator. However, they may be directly useful in
the creation of user-defined aggregates. Some examples:
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}}