Обсуждение: Index on array element

How can I create an index on an array element? I seem to recall having
done this in the past but I don't recall how.

steve=# \d foo
       Table "public.foo"
  Column   |  Type  | Modifiers
-----------+--------+-----------
 textarray | text[] |

steve=# create index foodex on foo (textarray[3]);
ERROR:  parser: parse error at or near "[" at character 38

steve=# select version();
PostgreSQL 7.3.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.3
20030226 (prerelease) (SuSE Linux)

Cheers,
Steve

Steve Crawford <scrawford@pinpointresearch.com> writes:
> How can I create an index on an array element?

You need 7.4 and an extra layer of parentheses:

    create index foodex on foo ((textarray[3]));

            regards, tom lane

Steve Crawford <scrawford@pinpointresearch.com> writes:

> How can I create an index on an array element? I seem to recall having
> done this in the past but I don't recall how.
>
> steve=# \d foo
>        Table "public.foo"
>   Column   |  Type  | Modifiers
> -----------+--------+-----------
>  textarray | text[] |
>
> steve=# create index foodex on foo (textarray[3]);
> ERROR:  parser: parse error at or near "[" at character 38
>
> steve=# select version();
> PostgreSQL 7.3.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.3
> 20030226 (prerelease) (SuSE Linux)

In 7.3 you'll have to create a function to make this index.

  create function first(text[]) returns text language sql as 'select $1 [1]' strict immutable;

then you can create an index like

  create index foodex on foo (first(textarray));

In 7.4 you can do arbitrary expressions, but in 7.3 you can only do simple
function calls of a single column.

--
greg

>> How can I create an index on an array element?
>
> You need 7.4 and an extra layer of parentheses:
>
>      create index foodex on foo ((textarray[3]));

        Sorry, but this isn't obvious to me as arrays in a database are a new
concept for me -- why are the extra layer of parenthesis needed?

--
Randolf Richardson - rr@8x.ca
Vancouver, British Columbia, Canada

Please do not eMail me directly when responding
to my postings in the newsgroups.

Randolf Richardson <rr@8x.ca> writes:
>>> How can I create an index on an array element?
>>
>> You need 7.4 and an extra layer of parentheses:
>>
>> create index foodex on foo ((textarray[3]));

>         Sorry, but this isn't obvious to me as arrays in a database are a new
> concept for me -- why are the extra layer of parenthesis needed?

It's got nothing to do with arrays as such, but is purely a syntactic
restriction: if the indexed item is anything but an unadorned column
name or a function call, CREATE INDEX wants parentheses around it.

This is because the CREATE INDEX syntax also allows for an "operator
class" name in there.  Consider

create index foodex on foo (bar ! ops);

Is this supposed to mean indexing the expression "bar ! ops" (infix !
operator) or indexing the expression "bar !" (postfix ! operator) using
the operator class "ops"?  No way to tell, so we make you clarify your
intent with parentheses.  You can say either

create index foodex on foo ((bar ! ops));
create index foodex on foo ((bar !) ops);

depending which interpretation you want.

            regards, tom lane

[sNip]
> It's got nothing to do with arrays as such, but is purely a syntactic
> restriction: if the indexed item is anything but an unadorned column
> name or a function call, CREATE INDEX wants parentheses around it.
>
> This is because the CREATE INDEX syntax also allows for an "operator
> class" name in there.  Consider
>
> create index foodex on foo (bar ! ops);
>
> Is this supposed to mean indexing the expression "bar ! ops" (infix !
> operator) or indexing the expression "bar !" (postfix ! operator) using
> the operator class "ops"?  No way to tell, so we make you clarify your
> intent with parentheses.  You can say either
>
> create index foodex on foo ((bar ! ops));
> create index foodex on foo ((bar !) ops);
>
> depending which interpretation you want.

        Thanks.  Your explanation is very helpful.

--
Randolf Richardson - rr@8x.ca
Vancouver, British Columbia, Canada

Please do not eMail me directly when responding
to my postings in the newsgroups.