Re: Forcing seq_scan off for large table joined with tiny table yeilds improved performance

Scott Marlowe wrote:
>> CREATE INDEX photo_info_data_ix_field_value
>> ON user_info_data USING btree (field_value);
>>
>> So, there is index on (user_id, field_name). Postgres is using index for
>> user_id (...WHERE user_id = 12345) but not on field-name (...WHERE
>> field_name = 'f-spot'). When I add extra index on field name:
>>
>> CREATE INDEX photo_info_data_ix__field_name
>> ON user_info_data USING btree (field_name);
>>
>> Then that index is used.
>
> On older versions of pgsql, the second of two terms in a multicolumn
> index can't be used alone. On newer versions it can, but it is much
> less efficient than if it's a single column index or if the term is
> the first one not the second.

I'm using 8.3.7. So, you'd also suggest to keep that extra (in a way
redundant) index on field_name, since I need PK on (photo_id, field_name) ?

Mike