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Small improvements to user's guide description of arrays.

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Tom Lane 2000-03-28 03:39:50 +00:00
parent 9dc12ef54f
commit 68c1801b1d
1 changed files with 71 additions and 15 deletions
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doc/src/sgml/array.sgml
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@ -10,14 +10,13 @@ This must become a chapter on array behavior. Volunteers? - thomas 1998-01-12
</Para>
<Para>
<ProductName>Postgres</ProductName> allows attributes of an instance to be defined
as fixed-length or variable-length multi-dimensional
arrays. Arrays of any base type or user-defined type
can be created. To illustrate their use, we first create a
class with arrays of base types.
<ProductName>Postgres</ProductName> allows attributes of a class
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 class:
<ProgramListing>
CREATE TABLE SAL_EMP (
CREATE TABLE sal_emp (
name text,
pay_by_quarter int4[],
schedule text[][]
@ -26,10 +25,10 @@ CREATE TABLE SAL_EMP (
</Para>
<Para>
The above query will create a class named SAL_EMP with
The above query will create a class named <FirstTerm>sal_emp</FirstTerm> with
a <FirstTerm>text</FirstTerm> string (name), a one-dimensional array of <FirstTerm>int4</FirstTerm>
(pay_by_quarter), which represents the employee's
salary by quarter and a two-dimensional array of <FirstTerm>text</FirstTerm>
salary by quarter, and a two-dimensional array of <FirstTerm>text</FirstTerm>
(schedule), which represents the employee's weekly
schedule. Now we do some <FirstTerm>INSERTS</FirstTerm>s; note that when
appending to an array, we enclose the values within
@ -37,20 +36,18 @@ CREATE TABLE SAL_EMP (
this is not unlike the syntax for initializing structures.
<ProgramListing>
INSERT INTO SAL_EMP
INSERT INTO sal_emp
VALUES ('Bill',
'{10000, 10000, 10000, 10000}',
'{{"meeting", "lunch"}, {}}');
INSERT INTO SAL_EMP
INSERT INTO sal_emp
VALUES ('Carol',
'{20000, 25000, 25000, 25000}',
'{{"talk", "consult"}, {"meeting"}}');
</ProgramListing>
By default, <ProductName>Postgres</ProductName> uses the "one-based" numbering
convention for arrays -- that is, an array of n elements starts with array[1] and ends with array[n].
Now, we can run some queries on SAL_EMP. First, we
Now, we can run some queries on sal_emp. 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:
@ -63,6 +60,10 @@ SELECT name FROM sal_emp WHERE pay_by_quarter[1] &lt;&gt; pay_by_quarter[2];
Carol
(1 row)
</ProgramListing>
<ProductName>Postgres</ProductName> uses the "one-based" numbering
convention for arrays --- that is, an array of n elements starts with
array[1] and ends with array[n].
</Para>
<Para>
@ -82,8 +83,10 @@ SELECT pay_by_quarter[3] FROM sal_emp;
<Para>
We can also access arbitrary slices of an array, or
subarrays. This query retrieves the first item on
Bill's schedule for the first two days of the week.
subarrays. An array slice is denoted by writing
"lower subscript : upper subscript" for one or more array
dimensions. This query retrieves the first item on
Bill's schedule for the first two days of the week:
<ProgramListing>
SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';
@ -93,6 +96,59 @@ SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';
{{"meeting"},{""}}
(1 row)
</ProgramListing>
We could also have written
<ProgramListing>
SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';
</ProgramListing>
with the same result.
</Para>
<Para>
An array value can be replaced completely:
<ProgramListing>
UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}'
WHERE name = 'Carol';
</ProgramListing>
or updated at a single entry:
<ProgramListing>
UPDATE sal_emp SET pay_by_quarter[4] = 15000
WHERE name = 'Bill';
</ProgramListing>
or updated in a slice:
<ProgramListing>
UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}'
WHERE name = 'Carol';
</ProgramListing>
</Para>
<Para>
It is not currently possible to resize an array value except by
complete replacement; for example, we couldn't change a four-
element array value to a five-element value with a single
assignment to array[5].
</Para>
<Para>
The syntax for CREATE TABLE allows fixed-length arrays to be
defined:
<ProgramListing>
CREATE TABLE tictactoe (
squares int4[3][3]
);
</ProgramListing>
However, the current implementation does not enforce the array
size limits --- the behavior is the same as for arrays of
unspecified length.
</Para>
</Chapter>