On 4/6/07, Michael Fuhr <mike@xxxxxxxx> wrote:
On Fri, Apr 06, 2007 at 04:38:33PM -0400, Alex Deucher wrote:
> One more anomaly between 7.4 and 8.2. DB dumped from 7.4 and loaded
> onto 8.2, both have locale set to C. 8.2 seems to prefer Seq Scans
> for the first query while the ordering in the second query seems to
> perform worse on 8.2. I ran analyze. I've tried with the encoding
> set to UTF-8 and SQL_ASCII; same numbers and plans. Any ideas how to
> improve this?
Are you sure the data sets are identical? The 7.4 query returned
0 rows; the 8.2 query returned 1 row. If you're running the same
query against the same data in both versions then at least one of
them appears to be returning the wrong result. Exactly which
versions of 7.4 and 8.2 are you running?
Have you analyzed all tables in both versions? The row count
estimate in 7.4 is much closer to reality than in 8.2:
7.4
> -> Index Scan using pnum_idx on event (cost=0.00..3.37 rows=19
> width=172) (actual time=0.063..0.063 rows=0 loops=1)
> Index Cond: ((pnum)::text = 'AB5819188'::text)
8.2
> -> Index Scan using pnum_idx on event (cost=0.00..3147.63
> rows=1779 width=171) (actual time=0.030..0.033 rows=1 loops=1)
> Index Cond: ((pnum)::text = 'AB5819188'::text)
If analyzing the event table doesn't improve the row count estimate
then try increasing the statistics target for event.pnum and analyzing
again. Example:
ALTER TABLE event ALTER pnum SET STATISTICS 100;
ANALYZE event;
You can set the statistics target as high as 1000 to get more
accurate results at the cost of longer ANALYZE times.
Setting statistics to 400 seems to be the sweet spot. Values above
that seem to only marginally improve performance. However, I have to
disable seqscan in order for the query to be fast. Why does the query
planner insist on doing a seq scan? Is there anyway to make it prefer
the index scan?
Thanks,
Alex
postgres 8.2
db=# EXPLAIN ANALYZE select pnum, event_pid, code_name,
code_description, code_mcam, event_date, effective_date, ref_country,
ref_country_legal_code, corresponding_pnum, withdrawal_date,
payment_date, extension_date, fee_payment_year, requester, free_form
from code inner join event on code_pid = code_pid_fk where pnum
='US5819188';
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=106.91..210.37 rows=54 width=229) (actual
time=11.245..11.253 rows=1 loops=1)
Hash Cond: (event.code_pid_fk = code.code_pid)
-> Index Scan using pnum_idx on event (cost=0.00..102.58 rows=54
width=170) (actual time=0.108..0.112 rows=1 loops=1)
Index Cond: ((pnum)::text = 'US5819188'::text)
-> Hash (cost=70.85..70.85 rows=2885 width=67) (actual
time=11.006..11.006 rows=2885 loops=1)
-> Seq Scan on code (cost=0.00..70.85 rows=2885 width=67)
(actual time=0.025..5.392 rows=2885 loops=1)
Total runtime: 11.429 ms
(7 rows)
db=# set enable_seqscan=0;
SET
db=# EXPLAIN ANALYZE select pnum, event_pid, code_name,
code_description, code_mcam, event_date, effective_date, ref_country,
ref_country_legal_code, corresponding_pnum, withdrawal_date,
payment_date, extension_date, fee_payment_year, requester, free_form
from code inner join event on code_pid = code_pid_fk where pnum
='US5819188';
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------
Nested Loop (cost=0.00..289.72 rows=54 width=229) (actual
time=0.068..0.076 rows=1 loops=1)
-> Index Scan using pnum_idx on event (cost=0.00..102.58 rows=54
width=170) (actual time=0.019..0.020 rows=1 loops=1)
Index Cond: ((pnum)::text = 'US5819188'::text)
-> Index Scan using code_pkey on code (cost=0.00..3.45 rows=1
width=67) (actual time=0.041..0.043 rows=1 loops=1)
Index Cond: (code.code_pid = event.code_pid_fk)
Total runtime: 0.126 ms
(6 rows)
