"Bruno BAGUETTE" <pgsql-ml@baguette.net> writes:
> Do you see a way to get better performances with this query which takes
> currently 655.07 msec to be done.
> levure=> explain analyze SELECT distinct lower(substr(l_name, 1, 1)) AS
> initiale FROM people
> levure-> UNION
> levure-> SELECT distinct lower(substr(org_name, 1, 1)) AS initiale FROM
> organizations
> levure-> ORDER BY initiale;
This is inherently a bit inefficient since the UNION implies a DISTINCT
step, thus partially repeating the DISTINCT work done inside each SELECT.
It would likely be a tad faster to drop the DISTINCTs from the
subselects and rely on UNION to do the filtering. However, you're still
gonna have a big SORT/UNIQUE step.
As of PG 7.4 you could probably get a performance win by converting the
thing to use GROUP BY instead of DISTINCT or UNION:
select initiale from (
select lower(substr(l_name,1,1)) as initiale from people
union all
select lower(substr(org_name,1,1)) as initiale from organizations
) ss
group by initiale order by initiale;
This should use a HashAggregate to do the unique-ification. I think
that will be faster than Sort/Unique.
regards, tom lane
