Обсуждение: Planner doing seqscan before indexed join
8.0.3 - Linux 2.6.18..
Freshly vacuumed and analyzed
This database has been humming along fine for a while now, but I've got one of those sticky queries that is taking
much too long to finish.
After some digging, I've found that the planner is choosing to apply a necessary seq scan to the table. Unfortunately,
it's scanning the whole table, when it seems that it could have joined it to a smaller table first and reduce the
amount of rows it would have to scan dramatically ( 70 million to about 5,000 ).
The table "eventactivity" has about 70million rows in it, index on "incidentid"
The table "keyword_incidents" is a temporary table and has incidentid as its primary key. It contains
5125 rows. eventmain and eventgeo both have 2.1 million. My hope is that I can convince the planner to do the
join to keyword_incidents *first* and then do the seq scan for the LIKE condition. Instead, it seems that it's
seqscanningthe
whole 70 million rows first and then doing the join, which takes a lot longer than I'd like to wait for it. Or, maybe
I'm
misreading the explain output?
Thanks again
-Dan
---------------------------------
Here's the query:
explain analyze
select
*
from
keyword_incidents,
eventactivity,
eventmain,
eventgeo
where
eventmain.incidentid = keyword_incidents.incidentid and
eventgeo.incidentid = keyword_incidents.incidentid and
eventactivity.incidentid = keyword_incidents.incidentid
and ( recordtext like '%JOSE CHAVEZ%' )
order by eventmain.entrydate limit 10000;
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=2388918.07..2388918.08 rows=1 width=455) (actual time=81771.186..81771.292 rows=26 loops=1)
-> Sort (cost=2388918.07..2388918.08 rows=1 width=455) (actual time=81771.180..81771.215 rows=26 loops=1)
Sort Key: eventmain.entrydate
-> Nested Loop (cost=0.00..2388918.06 rows=1 width=455) (actual time=357.389..81770.982 rows=26 loops=1)
-> Nested Loop (cost=0.00..2388913.27 rows=1 width=230) (actual time=357.292..81767.385 rows=26
loops=1)
-> Nested Loop (cost=0.00..2388909.33 rows=1 width=122) (actual time=357.226..81764.501 rows=26
loops=1)
-> Seq Scan on eventactivity (cost=0.00..2388874.46 rows=7 width=84) (actual
time=357.147..81762.582
rows=27 loops=1)
Filter: ((recordtext)::text ~~ '%JOSE CHAVEZ%'::text)
-> Index Scan using keyword_incidentid_pkey on keyword_incidents (cost=0.00..4.97 rows=1
width=38)
(actual time=0.034..0.036 rows=1 loops=27)
Index Cond: (("outer".incidentid)::text = (keyword_incidents.incidentid)::text)
-> Index Scan using eventgeo_incidentid_idx on eventgeo (cost=0.00..3.93 rows=1 width=108)
(actual
time=0.076..0.081 rows=1 loops=26)
Index Cond: (("outer".incidentid)::text = (eventgeo.incidentid)::text)
-> Index Scan using eventmain_incidentid_idx on eventmain (cost=0.00..4.78 rows=1 width=225) (actual
time=0.069..0.075 rows=1 loops=26)
Index Cond: (("outer".incidentid)::text = (eventmain.incidentid)::text)
Total runtime: 81771.529 ms
(15 rows)
> From: pgsql-performance-owner@postgresql.org > [mailto:pgsql-performance-owner@postgresql.org] On Behalf Of > Dan Harris > > After some digging, I've found that the planner is choosing > to apply a necessary seq scan to the table. Unfortunately, > it's scanning the whole table, when it seems that it could > have joined it to a smaller table first and reduce the > amount of rows it would have to scan dramatically ( 70 > million to about 5,000 ). > Joining will reduce the amount of rows to scan for the filter, but performing the join is non-trivial. If postgres is going to join two tables together without applying any filter first then it will have to do a seqscan of one of the tables, and if it chooses the table with 5000 rows, then it will have to do 5000 index scans on a table with 70 million records. I don't know which way would be faster. I wonder if you could find a way to use an index to do the text filter. Maybe tsearch2? I haven't used anything like that myself, maybe someone else has more input.
You may try to change the planner's opinion using sub queries. Something
like:
select * from
eventactivity,
(select * from
keyword_incidents,
eventmain,
eventgeo
where
eventmain.incidentid = keyword_incidents.incidentid
and eventgeo.incidentid = keyword_incidents.incidentid
and ( recordtext like '%JOSE CHAVEZ%' )
)foo
where eventactivity.incidentid = foo.incidentid
order by foo.entrydate limit 10000;
HTH,
Marc
-----Original Message-----
From: pgsql-performance-owner@postgresql.org
[mailto:pgsql-performance-owner@postgresql.org] On Behalf Of Dan Harris
Sent: Thursday, March 29, 2007 4:22 AM
To: PostgreSQL Performance
Subject: [PERFORM] Planner doing seqscan before indexed join
8.0.3 - Linux 2.6.18..
Freshly vacuumed and analyzed
This database has been humming along fine for a while now, but I've got
one of those sticky queries that is taking much too long to finish.
After some digging, I've found that the planner is choosing to apply a
necessary seq scan to the table. Unfortunately, it's scanning the whole
table, when it seems that it could have joined it to a smaller table
first and reduce the amount of rows it would have to scan dramatically (
70 million to about 5,000 ).
The table "eventactivity" has about 70million rows in it, index on
"incidentid"
The table "keyword_incidents" is a temporary table and has incidentid as
its primary key. It contains
5125 rows. eventmain and eventgeo both have 2.1 million. My hope is that
I can convince the planner to do the
join to keyword_incidents *first* and then do the seq scan for the
LIKE condition. Instead, it seems that it's seqscanning the whole 70
million rows first and then doing the join, which takes a lot longer
than I'd like to wait for it. Or, maybe I'm misreading the explain
output?
Thanks again
-Dan
---------------------------------
Here's the query:
explain analyze
select
* from
keyword_incidents,
eventactivity,
eventmain,
eventgeo
where
eventmain.incidentid = keyword_incidents.incidentid and
eventgeo.incidentid = keyword_incidents.incidentid and
eventactivity.incidentid = keyword_incidents.incidentid
and ( recordtext like '%JOSE CHAVEZ%' )
order by eventmain.entrydate limit 10000;
------------------------------------------------------------------------
------------------------------------------------------------------------
---------------------------
Limit (cost=2388918.07..2388918.08 rows=1 width=455) (actual
time=81771.186..81771.292 rows=26 loops=1)
-> Sort (cost=2388918.07..2388918.08 rows=1 width=455) (actual
time=81771.180..81771.215 rows=26 loops=1)
Sort Key: eventmain.entrydate
-> Nested Loop (cost=0.00..2388918.06 rows=1 width=455)
(actual time=357.389..81770.982 rows=26 loops=1)
-> Nested Loop (cost=0.00..2388913.27 rows=1
width=230) (actual time=357.292..81767.385 rows=26 loops=1)
-> Nested Loop (cost=0.00..2388909.33 rows=1
width=122) (actual time=357.226..81764.501 rows=26 loops=1)
-> Seq Scan on eventactivity
(cost=0.00..2388874.46 rows=7 width=84) (actual time=357.147..81762.582
rows=27 loops=1)
Filter: ((recordtext)::text ~~ '%JOSE
CHAVEZ%'::text)
-> Index Scan using keyword_incidentid_pkey
on keyword_incidents (cost=0.00..4.97 rows=1 width=38) (actual
time=0.034..0.036 rows=1 loops=27)
Index Cond:
(("outer".incidentid)::text = (keyword_incidents.incidentid)::text)
-> Index Scan using eventgeo_incidentid_idx on
eventgeo (cost=0.00..3.93 rows=1 width=108) (actual
time=0.076..0.081 rows=1 loops=26)
Index Cond: (("outer".incidentid)::text =
(eventgeo.incidentid)::text)
-> Index Scan using eventmain_incidentid_idx on
eventmain (cost=0.00..4.78 rows=1 width=225) (actual
time=0.069..0.075 rows=1 loops=26)
Index Cond: (("outer".incidentid)::text =
(eventmain.incidentid)::text)
Total runtime: 81771.529 ms
(15 rows)
---------------------------(end of broadcast)---------------------------
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