</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 TABLESAL_EMP (

CREATE TABLEsal_emp (

name text,

pay_by_quarter int4[],

schedule text[][]

</Para>

<Para>

The above query will create a class namedSAL_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

braces and separate them by commas. If you know <FirstTerm>C</FirstTerm>,

this is not unlike the syntax for initializing structures.

<ProgramListing>

INSERT INTOSAL_EMP

INSERT INTOsal_emp

VALUES ('Bill',

'{10000, 10000, 10000, 10000}',

'{{"meeting", "lunch"}, {}}');

INSERT INTOSAL_EMP

INSERT INTOsal_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:

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>

<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';

{{"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>