Re: [PERFORM] estimated rows vs. actual rows
After takin a swig o' Arrakan spice grog, [EMAIL PROTECTED] (Jaime Casanova) belched out: On Sun, 13 Feb 2005 13:41:09 -0800, Josh Berkus josh@agliodbs.com wrote: Jaime, Why is this query using a seq scan rather than a index scan? Because it thinks a seq scan will be faster. I will suggest him to probe with seq scans disabled. But, IMHO, if the table has 143902 and it thinks will retrieve 2610 (almost 1.81% of the total). it won't be faster with an index? If the 2610 rows are scattered widely enough, it may be cheaper to do a seq scan. After all, with a seq scan, you read each block of the table's pages exactly once. With an index scan, you read index pages _and_ table pages, and may do and redo some of the pages. It sounds as though it's worth forcing the matter and trying it both ways and comparing them. Don't be surprised if the seq scan is in fact faster... -- select 'cbbrowne' || '@' || 'gmail.com'; http://cbbrowne.com/info/emacs.html When aiming for the common denominator, be prepared for the occasional division by zero. ---(end of broadcast)--- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
[PERFORM] estimated rows vs. actual rows
Hi,
in the #postgresql-es channel someone shows me this:
pgsql-7.4.5 + postgis
--- begin context ---
CREATE TABLE calles (
gid int4 NOT NULL DEFAULT nextval('public.callesstgo_gid_seq'::text),
nombre varchar,
inicio int4,
termino int4,
comuna varchar,
ciudad varchar,
region numeric,
pais varchar,
the_geom geometry,
id_comuna numeric,
CONSTRAINT callesstgo_pkey PRIMARY KEY (gid),
CONSTRAINT enforce_geotype_the_geom CHECK (geometrytype(the_geom) =
'MULTILINESTRING'::text OR the_geom IS NULL),
CONSTRAINT enforce_srid_the_geom CHECK (srid(the_geom) = -1)
)
WITH OIDS;
CREATE INDEX idx_region_comunas ON calles USING btree
(id_comuna, region);
select count(*) from calles;
143902
--- end context ---
Ok . here is the problem (BTW, the database has been analyzed just
before this query was execured)
explain analyze
select * from calles where id_comuna = 92 and region=13;
QUERY PLAN Seq Scan on calles (cost=0.00..7876.53 rows=2610
width=279) (actual time=182.590..454.195 rows=4612 loops=1)
Filter: ((id_comuna = 92::numeric) AND (region = 13::numeric))
Total runtime: 456.876 ms
Why is this query using a seq scan rather than a index scan? i notice
the diff between the estimated rows and actual rows (almost 2000).
Can this affect the query plan? i think this is a problem of
statistics, am i right? if so, what can be done?
regards,
Jaime Casanova
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Re: [PERFORM] estimated rows vs. actual rows
Jaime, Why is this query using a seq scan rather than a index scan? Because it thinks a seq scan will be faster. i notice the diff between the estimated rows and actual rows (almost 2000). Yes, ANALYZE, and possibly increasing the column stats, should help that. Can this affect the query plan? i think this is a problem of statistics, am i right? if so, what can be done? Well, if the estimate was accurate, PG would be even *more* likely to use a seq scan (more rows). I think maybe you should establish whether a seq scan actually *is* faster? Perhaps do SET enable_seqscan = false and then re-run the query a few times? -- Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 7: don't forget to increase your free space map settings
Re: [PERFORM] estimated rows vs. actual rows
On Sun, 13 Feb 2005 13:41:09 -0800, Josh Berkus josh@agliodbs.com wrote: Jaime, Why is this query using a seq scan rather than a index scan? Because it thinks a seq scan will be faster. I will suggest him to probe with seq scans disabled. But, IMHO, if the table has 143902 and it thinks will retrieve 2610 (almost 1.81% of the total). it won't be faster with an index? i know, i will suggest him to probe to be sure. just an opinion. regards, Jaime Casanova ---(end of broadcast)--- TIP 7: don't forget to increase your free space map settings
Re: [PERFORM] estimated rows vs. actual rows
Jaime Casanova [EMAIL PROTECTED] writes: But, IMHO, if the table has 143902 and it thinks will retrieve 2610 (almost 1.81% of the total). it won't be faster with an index? That's almost one row in fifty. We don't know how wide the table is, but it's certainly possible that there are order-of-a-hundred rows on each page; in which case the indexscan is likely to hit every page. Twice. Not in sequence. Only if the selected rows are pretty well clustered in a small part of the table is this going to be a win over a seqscan. regards, tom lane ---(end of broadcast)--- TIP 7: don't forget to increase your free space map settings
Re: [PERFORM] estimated rows vs. actual rows
Jaime Casanova wrote: But, IMHO, if the table has 143902 and it thinks will retrieve 2610 (almost 1.81% of the total). it won't be faster with an index? Depends on how those 2610 rows are distributed amongst the 143902. The worst case scenario is each one of them in its own page. In that case you have to read 2610 *pages*, which is probably a significant percentage of the table. You can find out this information from the pg_stats view (particularly the correlation column). Mark ---(end of broadcast)--- TIP 7: don't forget to increase your free space map settings
