Re: query optimizer questions

Robert Berger <rwb(at)vtiscan(dot)com> writes:
> Why do the following three queries use different scan types under 7.1.2?

As Peter points out, index optimization of LIKE queries is disabled
unless your database is in C locale (if you're not sure,
contrib/pg_controldata can tell you what locale your database is using).

However, the estimated row counts seem peculiar to me in any case:

> anderson=# explain select value from s_n_grantor where value='foobar';
> NOTICE: QUERY PLAN:

> Index Scan using s_n_grantor_key on s_n_grantor (cost=0.00..823.58
> rows=247 width=12)

> EXPLAIN
> anderson=# explain select value from s_n_grantor where value like
> 'foobar';
> NOTICE: QUERY PLAN:

> Seq Scan on s_n_grantor (cost=0.00..8178.46 rows=247 width=12)

> anderson=# explain select value from s_n_grantor where value like
> 'foobar%';
> NOTICE: QUERY PLAN:

> Seq Scan on s_n_grantor (cost=0.00..8178.46 rows=1 width=12)

I'd expect the LIKE 'foobar' to produce the same estimate as = 'foobar',
because the LIKE estimator recognizes this as a fixed pattern. But why
is that considered *less* selective than LIKE 'foobar%'? Something
fishy about that. Could we see the output of

select attname,attdispersion,s.*
from pg_statistic s, pg_attribute a, pg_class c
where starelid = c.oid and attrelid = c.oid and staattnum = attnum
and relname = 's_n_grantor';

(If you get no output, it means you didn't VACUUM ANALYZE ...)

regards, tom lane