Re: TEXT column and indexing

On Wed, 19 Nov 2003, Ian Barwick wrote:

>
> I have this table:
>
> db=> \d object_property_value
> Table "db.object_property_value"
> Column | Type | Modifiers
> -----------------------+------------------------+--------------------
> obj_property_value_id | integer | not null default nextval(...
> obj_property_id | integer | not null
> value | text |
> Indexes:
> "object_property_value_pkey" primary key, btree (obj_property_value_id)
> "opv_obj_property_id_ix" btree (obj_property_id)
> "opv_v_ix" btree (substr(value, 1, 128))
> Foreign-key constraints:
> "object_property_fkey" FOREIGN KEY (obj_property_id)
> REFERENCES object_property(obj_property_id)
> ON UPDATE CASCADE ON DELETE CASCADE
> I want to query this table to match a specific value along
> the lines of:
>
> SELECT obj_property_id
> FROM object_property_value opv
> WHERE opv.value = 'foo'
>
> The question is therefore: can I get an index to work on the TEXT column? It
> is currently indexed with:
> "opv_v_ix" btree (substr(value, 1, 128))
>
> which doesn't appear to have any effect. I am probably missing something
> obvious though. I can live with maintaining an extra VARCHAR column but

You probably need to be querying like:
WHERE substr(value,1,128)='foo';
in order to use that index.

While substr(txtcol, 1,128) happens to have the property that it would be
probably be useful in a search against a short constant string, that's an
internal property of that function.