Thread: Re: Failure to use indexes (fwd)
Greetings all!
Given the quiet, I assume that there is no experience with index issues on
inherited tables? Just seeing if anybody may have any ideas or suggested
work arounds (I seem to have found one by constructing a query that does
all the joins between inherited tables explicitely - this causes the
indexes to be used - still trying to make sure it is a legitimate method).
Regards!
Ed
---------- Forwarded message ----------
Date: Fri, 29 Jul 2005 14:23:29 -0400 (EDT)
From: Edmund Dengler <edmundd@eSentire.com>
To: Postgresql-General <pgsql-general@postgresql.org>
Subject: Re: [GENERAL] Failure to use indexes
Greetings!
I managed to get an even simpler example:
select *
from eventlog.record_main
where luid in (
select luid from eventlog_partition._20050706__raw_record
order by luid limit 1
)
;
If I use <=> rather than <in>, postgresql uses index scanning. As soon as
I use <in> (ie, a set to compare against), I get sequential scanning,
event though the set size is only a single element.
Regards!
Ed
On Fri, 29 Jul 2005, Edmund Dengler wrote:
> Greetings!
>
> I am using <inherits> to partition several tables. When I perform a query
> on another table, and then try to join against an inherited table set, the
> optimizer does not use any indexes to perform the join.
>
> This is 8.0.1. Is this fixed in 8.0.2 or 8.0.3?
>
> The query:
>
> explain
> select *
> from (
> select * from eventlog.record_classification as record_classification
> where
> time_written >= '2005-07-06 00:00:00+00'::timestamptz
> and time_written < '2005-07-06 00:00:00+00'::timestamptz
> order by time_written, luid
> offset 0
> limit 500
> ) as classification
> join eventlog.record_main as main using (luid, time_written)
> ;
>
> The explanation:
>
> QUERY PLAN
>
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> Hash Join (cost=51.15..20191003.89 rows=208027 width=178)
> Hash Cond: (("outer".luid = "inner".luid) AND ("outer".time_written = "inner".time_written))
> -> Append (cost=0.00..14641488.64 rows=554738383 width=96)
> -> Seq Scan on record_main main (cost=0.00..0.00 rows=1 width=96)
> -> Seq Scan on _20050723__record_main main (cost=0.00..94078.62 rows=3564462 width=96)
> -> Seq Scan on _20050724__record_main main (cost=0.00..110075.12 rows=4170512 width=96)
> -> Seq Scan on _20050725__record_main main (cost=0.00..122836.02 rows=4654002 width=96)
> -> Seq Scan on _20050726__record_main main (cost=0.00..142347.71 rows=5393271 width=96)
> -> Seq Scan on _20050727__record_main main (cost=0.00..130858.80 rows=4957980 width=96)
> ....
> (and so on, currently 123 such inheritd tables)
> ....
> -> Hash (cost=51.07..51.07 rows=15 width=98)
> -> Subquery Scan classification (cost=50.89..51.07 rows=15 width=98)
> -> Limit (cost=50.89..50.92 rows=15 width=98)
> -> Sort (cost=50.89..50.92 rows=15 width=98)
> Sort Key: record_classification.time_written, record_classification.luid
> -> Result (cost=0.00..50.59 rows=15 width=98)
> -> Append (cost=0.00..50.59 rows=15 width=98)
> -> Seq Scan on record_classification (cost=0.00..0.00 rows=1 width=98)
> Filter: ((time_written >= '2005-07-06 00:00:00+00'::timestamp with time
zone)AND (time_written < '2005-07-06 00:00:00+00'::timestamp with time zone))
> -> Index Scan using _20050705__record_classification_time_written_idx on
_20050705__record_classificationrecord_classification (cost=0.00..3.46 rows=1 width=54)
> Index Cond: ((time_written >= '2005-07-06 00:00:00+00'::timestamp with
timezone) AND (time_written < '2005-07-06 00:00:00+00'::timestamp with time zone))
> -> Index Scan using _20050701__record_classification_time_written_idx on
_20050701__record_classificationrecord_classification (cost=0.00..3.59 rows=1 width=54)
> Index Cond: ((time_written >= '2005-07-06 00:00:00+00'::timestamp with
timezone) AND (time_written < '2005-07-06 00:00:00+00'::timestamp with time zone))
> -> Index Scan using _20050702__record_classification_time_written_idx on
_20050702__record_classificationrecord_classification (cost=0.00..3.69 rows=1 width=54)
> Index Cond: ((time_written >= '2005-07-06 00:00:00+00'::timestamp with
timezone) AND (time_written < '2005-07-06 00:00:00+00'::timestamp with time zone))
> -> Index Scan using _20050703__record_classification_time_written_idx on
_20050703__record_classificationrecord_classification (cost=0.00..3.70 rows=1 width=54)
> Index Cond: ((time_written >= '2005-07-06 00:00:00+00'::timestamp with
timezone) AND (time_written < '2005-07-06 00:00:00+00'::timestamp with time zone))
> ....
> (and so on)
> ....
> -> Index Scan using _20050714__record_classification_time_written_idx on
_20050714__record_classificationrecord_classification (cost=0.00..3.69 rows=1 width=53)
> Index Cond: ((time_written >= '2005-07-06 00:00:00+00'::timestamp with
timezone) AND (time_written < '2005-07-06 00:00:00+00'::timestamp with time zone))
> (164 rows)
>
> Sample tables:
>
> eventlog=# \d eventlog_partition._20050723__record_main
> Table "eventlog_partition._20050723__record_main"
> Column | Type | Modifiers
> ----------------------+--------------------------+-----------
> luid | bigint | not null
> host_luid | integer | not null
> log_luid | integer | not null
> time_logged | timestamp with time zone | not null
> record_number | bigint | not null
> time_generated_epoch | bigint | not null
> time_generated | timestamp with time zone | not null
> time_written_epoch | bigint | not null
> time_written | timestamp with time zone | not null
> event_id | bigint | not null
> event_type | integer | not null
> event_category | integer | not null
> source_luid | integer | not null
> computer_luid | integer | not null
> sid_luid | integer |
> message_luid | integer |
> Indexes:
> "_20050723__record_main_message_idx" UNIQUE, btree (message_luid, luid)
> "_20050723__record_main_sid_idx" UNIQUE, btree (sid_luid, luid)
> "_20050723__record_main_time_generated_idx" UNIQUE, btree (time_generated, luid)
> "_20050723__record_main_time_logged_idx" UNIQUE, btree (time_logged, luid)
> "_20050723__record_main_time_written_idx" UNIQUE, btree (time_written, luid)
> "_20050723__record_main_pkey" btree (luid)
> Inherits: record_main
>
>
> eventlog=# \d eventlog.record_classification
> Table "eventlog.record_classification"
> Column | Type | Modifiers
> ----------------+--------------------------+-----------
> luid | bigint | not null
> class_luid | integer | not null
> time_written | timestamp with time zone | not null
> account_luid | integer |
> group_luid | integer |
> caller_luid | integer |
> source_machine | character varying(30) |
> source_ip | character varying(30) |
> Indexes:
> "record_classification_pkey" PRIMARY KEY, btree (luid)
> "record_classification_account_idx" btree (account_luid, time_written)
> "record_classification_caller_idx" btree (caller_luid, time_written)
> "record_classification_class_idx" btree (class_luid, time_written)
> "record_classification_group_idx" btree (group_luid, time_written)
> "record_classification_ip_idx" btree (source_ip, time_written)
> "record_classification_machine_idx" btree (source_machine, time_written)
> Foreign-key constraints:
> "record_classification_class_luid_fkey" FOREIGN KEY (class_luid) REFERENCES eventlog.classification(luid)
> "record_classification_account_luid_fkey" FOREIGN KEY (account_luid) REFERENCES eventlog.account(luid)
> "record_classification_group_luid_fkey" FOREIGN KEY (group_luid) REFERENCES eventlog.account(luid)
> "record_classification_caller_luid_fkey" FOREIGN KEY (caller_luid) REFERENCES eventlog.account(luid)
>
>
> <luid> represents a unique key. I have tried the query using just <luid>
> as the join condition, same result.
>
> The system can use the <_XXXXXXXX__record_main_pkey> index on each
> <_XXXXXXXX__record_main> table to do an index scan on <luid>, but instead
> it chooses to do a sequential scan. This is true whether
> <_XXXXXXXX__record_main_pkey> is specified as unique or not.
>
> I have "vacuum analyze" the entire database before running the queries. I
> have set the <default_statistics_target> in postgresql.conf to 100 to
> obtain more accurate statistics.
>
> If I specify a specific sub-table (ie,
> <eventlog_partition._XXXXXXXX__record_main>), in the join, the optimizer
> uses the indexes to speed search.
>
> Any ideas?
>
> Regards!
> Ed
>
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The solution to my problem was to increase statistics
value and do another analyze. You can also change
default_statistics_target parameter in
postgresql.conf. Don't know if that's related to the
problem you're seeing, but it's worth a try.
Eugene
--- Edmund Dengler <edmundd@eSentire.com> wrote:
> Greetings all!
>
> Given the quiet, I assume that there is no
> experience with index issues on
> inherited tables? Just seeing if anybody may have
> any ideas or suggested
> work arounds (I seem to have found one by
> constructing a query that does
> all the joins between inherited tables explicitely -
> this causes the
> indexes to be used - still trying to make sure it is
> a legitimate method).
>
> Regards!
> Ed
>
> ---------- Forwarded message ----------
> Date: Fri, 29 Jul 2005 14:23:29 -0400 (EDT)
> From: Edmund Dengler <edmundd@eSentire.com>
> To: Postgresql-General
> <pgsql-general@postgresql.org>
> Subject: Re: [GENERAL] Failure to use indexes
>
> Greetings!
>
> I managed to get an even simpler example:
>
> select *
> from eventlog.record_main
> where luid in (
> select luid from
> eventlog_partition._20050706__raw_record
> order by luid limit 1
> )
> ;
>
> If I use <=> rather than <in>, postgresql uses index
> scanning. As soon as
> I use <in> (ie, a set to compare against), I get
> sequential scanning,
> event though the set size is only a single element.
>
> Regards!
> Ed
>
>
> On Fri, 29 Jul 2005, Edmund Dengler wrote:
>
> > Greetings!
> >
> > I am using <inherits> to partition several tables.
> When I perform a query
> > on another table, and then try to join against an
> inherited table set, the
> > optimizer does not use any indexes to perform the
> join.
> >
> > This is 8.0.1. Is this fixed in 8.0.2 or 8.0.3?
> >
> > The query:
> >
> > explain
> > select *
> > from (
> > select * from eventlog.record_classification
> as record_classification
> > where
> > time_written >= '2005-07-06
> 00:00:00+00'::timestamptz
> > and time_written < '2005-07-06
> 00:00:00+00'::timestamptz
> > order by time_written, luid
> > offset 0
> > limit 500
> > ) as classification
> > join eventlog.record_main as main using (luid,
> time_written)
> > ;
> >
> > The explanation:
> >
> >
> QUERY
> PLAN
> >
>
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> > Hash Join (cost=51.15..20191003.89 rows=208027
> width=178)
> > Hash Cond: (("outer".luid = "inner".luid) AND
> ("outer".time_written = "inner".time_written))
> > -> Append (cost=0.00..14641488.64
> rows=554738383 width=96)
> > -> Seq Scan on record_main main
> (cost=0.00..0.00 rows=1 width=96)
> > -> Seq Scan on _20050723__record_main
> main (cost=0.00..94078.62 rows=3564462 width=96)
> > -> Seq Scan on _20050724__record_main
> main (cost=0.00..110075.12 rows=4170512 width=96)
> > -> Seq Scan on _20050725__record_main
> main (cost=0.00..122836.02 rows=4654002 width=96)
> > -> Seq Scan on _20050726__record_main
> main (cost=0.00..142347.71 rows=5393271 width=96)
> > -> Seq Scan on _20050727__record_main
> main (cost=0.00..130858.80 rows=4957980 width=96)
> > ....
> > (and so on, currently 123 such inheritd
> tables)
> > ....
> > -> Hash (cost=51.07..51.07 rows=15 width=98)
> > -> Subquery Scan classification
> (cost=50.89..51.07 rows=15 width=98)
> > -> Limit (cost=50.89..50.92
> rows=15 width=98)
> > -> Sort (cost=50.89..50.92
> rows=15 width=98)
> > Sort Key:
> record_classification.time_written,
> record_classification.luid
> > -> Result
> (cost=0.00..50.59 rows=15 width=98)
> > -> Append
> (cost=0.00..50.59 rows=15 width=98)
> > -> Seq
> Scan on record_classification (cost=0.00..0.00
> rows=1 width=98)
> >
> Filter: ((time_written >= '2005-07-06
> 00:00:00+00'::timestamp with time zone) AND
> (time_written < '2005-07-06 00:00:00+00'::timestamp
> with time zone))
> > -> Index
> Scan using
> _20050705__record_classification_time_written_idx on
> _20050705__record_classification
> record_classification (cost=0.00..3.46 rows=1
> width=54)
> > Index
> Cond: ((time_written >= '2005-07-06
> 00:00:00+00'::timestamp with time zone) AND
> (time_written < '2005-07-06 00:00:00+00'::timestamp
> with time zone))
> > -> Index
> Scan using
> _20050701__record_classification_time_written_idx on
> _20050701__record_classification
> record_classification (cost=0.00..3.59 rows=1
> width=54)
> > Index
> Cond: ((time_written >= '2005-07-06
> 00:00:00+00'::timestamp with time zone) AND
> (time_written < '2005-07-06 00:00:00+00'::timestamp
> with time zone))
> > -> Index
> Scan using
> _20050702__record_classification_time_written_idx on
> _20050702__record_classification
> record_classification (cost=0.00..3.69 rows=1
> width=54)
> > Index
> Cond: ((time_written >= '2005-07-06
> 00:00:00+00'::timestamp with time zone) AND
> (time_written < '2005-07-06 00:00:00+00'::timestamp
> with time zone))
> > -> Index
> Scan using
> _20050703__record_classification_time_written_idx on
> _20050703__record_classification
> record_classification (cost=0.00..3.70 rows=1
> width=54)
> > Index
> Cond: ((time_written >= '2005-07-06
> 00:00:00+00'::timestamp with time zone) AND
> (time_written < '2005-07-06 00:00:00+00'::timestamp
> with time zone))
> > ....
> > (and so on)
> > ....
> > -> Index
> Scan using
> _20050714__record_classification_time_written_idx on
> _20050714__record_classification
> record_classification (cost=0.00..3.69 rows=1
> width=53)
> > Index
> Cond: ((time_written >= '2005-07-06
> 00:00:00+00'::timestamp with time zone) AND
> (time_written < '2005-07-06 00:00:00+00'::timestamp
> with time zone))
> > (164 rows)
> >
> > Sample tables:
> >
> > eventlog=# \d
> eventlog_partition._20050723__record_main
> > Table
> "eventlog_partition._20050723__record_main"
> > Column | Type |
> Modifiers
> >
>
----------------------+--------------------------+-----------
> > luid | bigint |
> not
=== message truncated ===
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On Tue, 2005-08-02 at 16:06, Dr NoName wrote: > The solution to my problem was to increase statistics > value and do another analyze. You can also change > default_statistics_target parameter in > postgresql.conf. Don't know if that's related to the > problem you're seeing, but it's worth a try. Cool postgresql trick: alter database test set default_statistics_target=200; You can change the default for a databases's new tables too.
Greetings! I have already increased the stats from 10 to 100. In addition, if I specify individual tables, then the indexes are used. However, when I go through the <inherits>, then indexes are not used. I will try and expand the statistics, but suspect it is not the root cause of the problem. Regards! Ed On Tue, 2 Aug 2005, Scott Marlowe wrote: > On Tue, 2005-08-02 at 16:06, Dr NoName wrote: > > The solution to my problem was to increase statistics > > value and do another analyze. You can also change > > default_statistics_target parameter in > > postgresql.conf. Don't know if that's related to the > > problem you're seeing, but it's worth a try. > > Cool postgresql trick: > > alter database test set default_statistics_target=200; > > You can change the default for a databases's new tables too. > > ---------------------------(end of broadcast)--------------------------- > TIP 2: Don't 'kill -9' the postmaster >
post your table definitions. I suspect you are indexing the parent table but not the children. btw, we tried using inherited tables in our application and quickly found out that they are more trouble then they are worth (at least the way they are implemented in postgresql). There are other, more portable ways of mapping a class hierarchy to table(s). A few techniques are described in Fowler's Patterns of Enterprise Application Architecture. hope this helps, Eugene --- Edmund Dengler <edmundd@eSentire.com> wrote: > Greetings! > > I have already increased the stats from 10 to 100. > In addition, if I > specify individual tables, then the indexes are > used. However, when I go > through the <inherits>, then indexes are not used. I > will try and expand > the statistics, but suspect it is not the root cause > of the problem. > > Regards! > Ed > > > On Tue, 2 Aug 2005, Scott Marlowe wrote: > > > On Tue, 2005-08-02 at 16:06, Dr NoName wrote: > > > The solution to my problem was to increase > statistics > > > value and do another analyze. You can also > change > > > default_statistics_target parameter in > > > postgresql.conf. Don't know if that's related to > the > > > problem you're seeing, but it's worth a try. > > > > Cool postgresql trick: > > > > alter database test set > default_statistics_target=200; > > > > You can change the default for a databases's new > tables too. > > > > ---------------------------(end of > broadcast)--------------------------- > > TIP 2: Don't 'kill -9' the postmaster > > > > ---------------------------(end of > broadcast)--------------------------- > TIP 2: Don't 'kill -9' the postmaster > __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
Greetings! Table definitions were included in the original post. I can repost (or send directly) if desired. I am using <inherits> to implement database partitioning based on the day. Postgresql 8.0.1 (and previous) has a number of issues when it comes to very large tables. Currently we have anywhere from 4,000,000 to 10,000,000 rows per day to deal with. With a <vacuum> running, we can't seem to cancel it until it finishes (which takes a bit of time when you have over 3 months of data). Insert/search performance also degrades with one large table (this gets into the issues of dealing with large volumes of time series data - really wish Postgresql would have the concept of a queue table where rows are always inserted at the end, and deletes only happen at the beginning, with block allocation). By using <inherits>, we can truncate a days worth of data very quickly, and only vacuum changing tables. Hopefully, the use of constraints in the query optimizer will make it into 8.1, so it will help some of our queries. I could hand optimize queries, but then I am essentially implementing an optimizer in our application code, which is definitely the wrong place to put it (also, if I was to go the full way, then I might switch to a database that supports time series data better, but would lose the ad-hoc query abilities of SQL). Indexes are on all the children (as per the post). in addition, when I use child tables directly, the indexes are used in the queries (which is one of the reasons why I suspect a bug related to pushing optimization information through <inherits>). Note that I also posted a followup that showed how a 1 row set would not use indexes when going through <inherits> whereas a simple = would. Regards! Ed On Wed, 3 Aug 2005, Dr NoName wrote: > post your table definitions. I suspect you are > indexing the parent table but not the children. > > btw, we tried using inherited tables in our > application and quickly found out that they are more > trouble then they are worth (at least the way they are > implemented in postgresql). There are other, more > portable ways of mapping a class hierarchy to > table(s). A few techniques are described in Fowler's > Patterns of Enterprise Application Architecture. > > hope this helps, > > Eugene > > > --- Edmund Dengler <edmundd@eSentire.com> wrote: > > > Greetings! > > > > I have already increased the stats from 10 to 100. > > In addition, if I > > specify individual tables, then the indexes are > > used. However, when I go > > through the <inherits>, then indexes are not used. I > > will try and expand > > the statistics, but suspect it is not the root cause > > of the problem. > > > > Regards! > > Ed > > > > > > On Tue, 2 Aug 2005, Scott Marlowe wrote: > > > > > On Tue, 2005-08-02 at 16:06, Dr NoName wrote: > > > > The solution to my problem was to increase > > statistics > > > > value and do another analyze. You can also > > change > > > > default_statistics_target parameter in > > > > postgresql.conf. Don't know if that's related to > > the > > > > problem you're seeing, but it's worth a try. > > > > > > Cool postgresql trick: > > > > > > alter database test set > > default_statistics_target=200; > > > > > > You can change the default for a databases's new > > tables too. > > > > > > ---------------------------(end of > > broadcast)--------------------------- > > > TIP 2: Don't 'kill -9' the postmaster > > > > > > > ---------------------------(end of > > broadcast)--------------------------- > > TIP 2: Don't 'kill -9' the postmaster > > > > > __________________________________________________ > Do You Yahoo!? > Tired of spam? Yahoo! Mail has the best spam protection around > http://mail.yahoo.com > > ---------------------------(end of broadcast)--------------------------- > TIP 1: if posting/reading through Usenet, please send an appropriate > subscribe-nomail command to majordomo@postgresql.org so that your > message can get through to the mailing list cleanly >