Обсуждение: Wrong rows count estimation (explain, gist, tsearch)
Hi, community
I have a table containing column for FTS and an appropriate index:
zzz=# \d search_table
...
obj_tsvector | tsvector |
not null default ''::tsvector
...
"i_search_table__tsvector_1" gist (obj_tsvector) WHERE obj_status_did = 1
The table filled with about 7.5E+6 rows. Most of them have different
from default values in obj_tsvector column. I use "estimated rows
count trick" to make search results counter faster, and every time
when obj_tsvector is used estimation rows count is extremely differ
from actual (eg. 6821 vs 372012). I played with SET STATISTICS but
have no success.
zzz=# EXPLAIN ANALYZE SELECT count(1) FROM search_table WHERE
obj_status_did = 1 AND obj_tsvector @@ (make_tsquery('(музыка)',
'utf8_russian'));
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=25226.63..25226.64 rows=1 width=0) (actual
time=14832.455..14832.455 rows=1 loops=1)
-> Bitmap Heap Scan on search_table (cost=465.16..25209.57
rows=6821 width=0) (actual time=3202.390..14731.096 rows=371026
loops=1)
Recheck Cond: (obj_status_did = 1)
Filter: (obj_tsvector @@ '''музыка'''::tsquery)
-> Bitmap Index Scan on i_search_table__tsvector_1
(cost=0.00..463.45 rows=6821 width=0) (actual time=2919.257..2919.257
rows=372012 loops=1)
Index Cond: (obj_tsvector @@ '''музыка'''::tsquery)
Total runtime: 14832.555 ms
(7 rows)
PG version - 8.3.7, STATISTICS is set to 500 for the column.
What's wrong with it? Is it possible to solve the problem? Thanx.
--
Regards,
Sergey Konoplev
BTW, dead tupples <5%
On Mon, Sep 28, 2009 at 11:09 AM, Sergey Konoplev <gray.ru@gmail.com> wrote:
> Hi, community
>
> I have a table containing column for FTS and an appropriate index:
>
> zzz=# \d search_table
> ...
> obj_tsvector | tsvector |
> not null default ''::tsvector
> ...
> "i_search_table__tsvector_1" gist (obj_tsvector) WHERE obj_status_did = 1
>
>
> The table filled with about 7.5E+6 rows. Most of them have different
> from default values in obj_tsvector column. I use "estimated rows
> count trick" to make search results counter faster, and every time
> when obj_tsvector is used estimation rows count is extremely differ
> from actual (eg. 6821 vs 372012). I played with SET STATISTICS but
> have no success.
>
> zzz=# EXPLAIN ANALYZE SELECT count(1) FROM search_table WHERE
> obj_status_did = 1 AND obj_tsvector @@ (make_tsquery('(музыка)',
> 'utf8_russian'));
>
> QUERY PLAN
>
---------------------------------------------------------------------------------------------------------------------------------------------------
> Aggregate (cost=25226.63..25226.64 rows=1 width=0) (actual
> time=14832.455..14832.455 rows=1 loops=1)
> -> Bitmap Heap Scan on search_table (cost=465.16..25209.57
> rows=6821 width=0) (actual time=3202.390..14731.096 rows=371026
> loops=1)
> Recheck Cond: (obj_status_did = 1)
> Filter: (obj_tsvector @@ '''музыка'''::tsquery)
> -> Bitmap Index Scan on i_search_table__tsvector_1
> (cost=0.00..463.45 rows=6821 width=0) (actual time=2919.257..2919.257
> rows=372012 loops=1)
> Index Cond: (obj_tsvector @@ '''музыка'''::tsquery)
> Total runtime: 14832.555 ms
> (7 rows)
>
> PG version - 8.3.7, STATISTICS is set to 500 for the column.
>
> What's wrong with it? Is it possible to solve the problem? Thanx.
>
> --
> Regards,
> Sergey Konoplev
>
--
Regards,
Sergey Konoplev
--
PostgreSQL articles in english & russian
http://gray-hemp.blogspot.com/search/label/postgresql/
Sergey Konoplev <gray.ru@gmail.com> writes:
> The table filled with about 7.5E+6 rows. Most of them have different
> from default values in obj_tsvector column. I use "estimated rows
> count trick" to make search results counter faster, and every time
> when obj_tsvector is used estimation rows count is extremely differ
> from actual (eg. 6821 vs 372012). I played with SET STATISTICS but
> have no success.
8.3 has just a stub estimator for @@. You might have better results
with 8.4. In the particular example you're showing, though, I don't
think the poor rowcount estimate is making any difference to the
plan choice.
regards, tom lane
On Mon, Sep 28, 2009 at 6:26 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote: > Sergey Konoplev <gray.ru@gmail.com> writes: >> The table filled with about 7.5E+6 rows. Most of them have different >> from default values in obj_tsvector column. I use "estimated rows >> count trick" to make search results counter faster, and every time >> when obj_tsvector is used estimation rows count is extremely differ >> from actual (eg. 6821 vs 372012). I played with SET STATISTICS but >> have no success. > > 8.3 has just a stub estimator for @@. You might have better results > with 8.4. In the particular example you're showing, though, I don't > think the poor rowcount estimate is making any difference to the > plan choice. Thanx, Tom. Will try 8.4 > > regards, tom lane > -- Regards, Sergey Konoplev -- PostgreSQL articles in english & russian http://gray-hemp.blogspot.com/search/label/postgresql/