Sezai YILMAZ wrote:
Hello!
I have a table eith name person as described below. It has an unique
index for id column (it is also primary key) and has an index for parent
column.
If I run a query with where clause on id column it uses the index (look
at the first explain analyze result; it says "Index Scan using...") and
the query for 582856 rows table results in 225,893 ms.
But, if I run another query with where clause on parent column it does
not use the index (look at the second explain analyze result; it says
"Seq Scan using...") and the query for 582856 rows table results in
11192.913 ms.
Why the difference of both queries is so dramatical for unique and
non-unique indexed columns? Why PostgreSQL does not use the non-unique
indexes (it says that it does sequential scan)?
Because it thinks it will be faster/cheaper.
I have to use an index on non-unique column. What is the solution for
that? Is there a way to speed up non-unique indexes?
You don't want to force it to use an index, you want it to make better
estimates. Let's have a look...
*****************************************************************
test=> \d person
Table "public.person"
Column | Type | Modifiers
---------+-----------------------+-----------
name | character varying(30) |
surname | character varying(30) |
id | integer | not null
parent | integer |
Indexes:
"person_pkey" primary key, btree (id)
"parent_ndx" btree (parent)
OK - all very simple. And you've said there are about 580,000 rows.
test=> explain analyze select id,name from person where id in ('17201',
'338191', '244319', '515209', '20415');
Why are you quoting integers?
test=> explain analyze select * from person where parent in ('17201',
'338191', '244319', '515209', '20415');
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
Seq Scan on person (cost=0.00..35871.26 rows=14427 width=32) (actual
time=0.063..11192.809 rows=5 loops=1)
Filter: ((parent = 17201) OR (parent = 338191) OR (parent = 244319) OR
(parent = 515209) OR (parent = 20415))
Total runtime: 11192.913 ms
Hmm - for some reason it's expecting 14427 rows to be returned. If there
were that many matches, then it might well be a better choice than going
back and fore between the index and the table all the time.
So - we need to find out why it thinks there will be so many rows returned.
1. VACUUM FULL ANALYSE person;
2. re-run the explain
That will make sure that the table's statistics are up-to-date. If that
hasn't helped, then perhaps we need to educate PG about the distribution
of values in "parent".
2. ALTER TABLE person ALTER COLUMN parent SET STATISTICS 100;
3. ANALYSE person;
4. re-run the explain
If that still doesn't work, keep increasing the statistics (max. value
1000).
Let us know how that works for you.
--
Richard Huxton
Archonet Ltd
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