Thread: Why isn't this index being used?

Why isn't this index being used?

From

"Knutsen, Mark"

Date:

19 October 2004, 19:15:21

The following is from a database of several hundred million rows of real data that has been VACUUM ANALYZEd.

Why isn't the index being used for a query that seems tailor-made for it? The results (6,300 rows) take about ten minutes to retrieve with a sequential scan.

A copy of this database with "integer" in place of "smallint", a primary key in column order (date, time, type, subtype) and a secondary index in the required order (type, subtype, date, time) correctly uses the secondary index to return results in under a second.

Actually, the integer version is the first one I made, and the smallint is the copy, but that shouldn't matter.

Postgres is version "postgresql-server-7.3.4-3.rhl9" from Red Hat Linux 9.

=====

testdb2=# \d db

Table "public.db"

Column | Type | Modifiers

---------+------------------------+-----------

date | date | not null

time | time without time zone | not null

type | smallint | not null

subtype | smallint | not null

value | integer |

Indexes: db_pkey primary key btree ("type", subtype, date, "time")

testdb2=# set enable_seqscan to off;

SET

testdb2=# explain select * from db where type=90 and subtype=70 and date='7/1/2004';

QUERY PLAN

------------------------------------------------------------------------------

Seq Scan on db (cost=100000000.00..107455603.76 rows=178 width=20)

Filter: (("type" = 90) AND (subtype = 70) AND (date = '2004-07-01'::date))

(2 rows)

Re: Why isn't this index being used?

From

Doug Y

Date:

19 October 2004, 19:28:37

Hi, I ran into a similar problem using bigints...

See:
http://www.postgresql.org/docs/7.3/static/datatype.html#DATATYPE-INT

small & big int have to be cast when used in querries... try:
explain select * from db where type=90::smallint and
subtype=70::smallint and date='7/1/2004';
or
explain select * from db where type='90' and subtype='70' and
date='7/1/2004';

Knutsen, Mark wrote:

> The following is from a database of several hundred million rows of
> real data that has been VACUUM ANALYZEd.
>
>
>
> Why isn't the index being used for a query that seems tailor-made for
> it? The results (6,300 rows) take about ten minutes to retrieve with a
> sequential scan.
>
>
>
> A copy of this database with "integer" in place of "smallint", a
> primary key in column order (date, time, type, subtype) and a
> secondary index in the required order (type, subtype, date, time)
> correctly uses the secondary index to return results in under a second.
>
>
>
> Actually, the integer version is the first one I made, and the
> smallint is the copy, but that shouldn't matter.
>
>
>
> Postgres is version "postgresql-server-7.3.4-3.rhl9" from Red Hat Linux 9.
>
>
>
> =====
>
>
>
> testdb2=# \d db
>
>               Table "public.db"
>
>  Column  |          Type          | Modifiers
>
> ---------+------------------------+-----------
>
>  date    | date                   | not null
>
>  time    | time without time zone | not null
>
>  type    | smallint               | not null
>
>  subtype | smallint               | not null
>
>  value   | integer                |
>
> Indexes: db_pkey primary key btree ("type", subtype, date, "time")
>
>
>
> testdb2=# set enable_seqscan to off;
>
> SET
>
>
>
> testdb2=# explain select * from db where type=90 and subtype=70 and
> date='7/1/2004';
>
>                                   QUERY PLAN
>
> ------------------------------------------------------------------------------
>
>  Seq Scan on db  (cost=100000000.00..107455603.76 rows=178 width=20)
>
>    Filter: (("type" = 90) AND (subtype = 70) AND (date =
> '2004-07-01'::date))
>
> (2 rows)
>

Re: Why isn't this index being used?

From

"Knutsen, Mark"

Date:

19 October 2004, 19:34:10

(Why don't replies automatically go to the list?)

Sure enough, quoting the constants fixes the problem.

Is it a best practice to always quote constants?

> -----Original Message-----
> From: Doug Y [mailto:dylists@ptd.net]
> Sent: Tuesday, October 19, 2004 11:28 AM
> To: Knutsen, Mark
> Cc: pgsql-performance@postgresql.org
> Subject: Re: [PERFORM] Why isn't this index being used?
>
> Hi, I ran into a similar problem using bigints...
>
> See:
> http://www.postgresql.org/docs/7.3/static/datatype.html#DATATYPE-INT
>
> small & big int have to be cast when used in querries... try:
> explain select * from db where type=90::smallint and
> subtype=70::smallint and date='7/1/2004';
> or
> explain select * from db where type='90' and subtype='70' and
> date='7/1/2004';
>
> Knutsen, Mark wrote:
>
> > The following is from a database of several hundred million rows of
> > real data that has been VACUUM ANALYZEd.
> >
> >
> >
> > Why isn't the index being used for a query that seems tailor-made
for
> > it? The results (6,300 rows) take about ten minutes to retrieve with
a
> > sequential scan.
> >
> >
> >
> > A copy of this database with "integer" in place of "smallint", a
> > primary key in column order (date, time, type, subtype) and a
> > secondary index in the required order (type, subtype, date, time)
> > correctly uses the secondary index to return results in under a
second.
> >
> >
> >
> > Actually, the integer version is the first one I made, and the
> > smallint is the copy, but that shouldn't matter.
> >
> >
> >
> > Postgres is version "postgresql-server-7.3.4-3.rhl9" from Red Hat
Linux
> 9.
> >
> >
> >
> > =====
> >
> >
> >
> > testdb2=# \d db
> >
> >               Table "public.db"
> >
> >  Column  |          Type          | Modifiers
> >
> > ---------+------------------------+-----------
> >
> >  date    | date                   | not null
> >
> >  time    | time without time zone | not null
> >
> >  type    | smallint               | not null
> >
> >  subtype | smallint               | not null
> >
> >  value   | integer                |
> >
> > Indexes: db_pkey primary key btree ("type", subtype, date, "time")
> >
> >
> >
> > testdb2=# set enable_seqscan to off;
> >
> > SET
> >
> >
> >
> > testdb2=# explain select * from db where type=90 and subtype=70 and
> > date='7/1/2004';
> >
> >                                   QUERY PLAN
> >
> >
------------------------------------------------------------------------
> ------
> >
> >  Seq Scan on db  (cost=100000000.00..107455603.76 rows=178 width=20)
> >
> >    Filter: (("type" = 90) AND (subtype = 70) AND (date =
> > '2004-07-01'::date))
> >
> > (2 rows)

Re: Why isn't this index being used?

From

Andrew Sullivan

Date:

19 October 2004, 19:47:27

On Tue, Oct 19, 2004 at 11:33:50AM -0400, Knutsen, Mark wrote:
> (Why don't replies automatically go to the list?)

Because sometimes you don't want them to.  There's been dozens of
discussions about this.  BTW, mutt has a nice feature which allows
you to reply to lists -- I imagine other MUAs have such a feature
too.

> Sure enough, quoting the constants fixes the problem.
>
> Is it a best practice to always quote constants?

No, but it's very useful in these cases.  The problem is I believe
this is fixed in 8.0, BTW.  See the FAQ, question 4.8

A

--
Andrew Sullivan  | ajs@crankycanuck.ca
I remember when computers were frustrating because they *did* exactly what
you told them to.  That actually seems sort of quaint now.
        --J.D. Baldwin