Re: [PERFORM] Index order ignored after `is null` in query
On 11/7/14, 5:14 AM, Artūras Lapinskas wrote: thanks for your time and answer. Not treating IS NULL as equality operator definitely helps me to make more sense out of previous explains. You can also try creating a partial index WHERE b IS NULL. WHERE b IS NOT NULL can also sometimes be useful, though for different reasons. -- Jim Nasby, Data Architect, Blue Treble Consulting Data in Trouble? Get it in Treble! http://BlueTreble.com -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Index order ignored after `is null` in query
Hi, thanks for your time and answer. Not treating IS NULL as equality operator definitely helps me to make more sense out of previous explains. -- Best Regard, Artūras Lapinskas On Thu, Nov 06, 2014 at 12:23:12PM -0500, Tom Lane wrote: =?utf-8?Q?Art=C5=ABras?= Lapinskas writes: After some more investigation my wild guess would be that then nulls are involved in query postgresql wants to double check whatever they are really nulls in actual relation (maybe because of dead tuples). No, it's much simpler than that: IS NULL is not an equality operator, so it's not treated as constraining sort order. What you're asking for amounts to building in an assumption that "all nulls are equal", which is exactly not what the SQL semantics for NULL say. So I feel that you have probably chosen a bogus data design that is misusing NULL for a purpose at variance with the SQL semantics. That's likely to bite you on the rear in many more ways than this. Even disregarding the question of whether it's semantically appropriate, getting the planner to handle IS NULL this way would be a significant amount of work. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Index order ignored after `is null` in query
=?utf-8?Q?Art=C5=ABras?= Lapinskas writes: > After some more investigation my wild guess would be that then nulls are > involved in query postgresql wants to double check whatever they are > really nulls in actual relation (maybe because of dead tuples). No, it's much simpler than that: IS NULL is not an equality operator, so it's not treated as constraining sort order. What you're asking for amounts to building in an assumption that "all nulls are equal", which is exactly not what the SQL semantics for NULL say. So I feel that you have probably chosen a bogus data design that is misusing NULL for a purpose at variance with the SQL semantics. That's likely to bite you on the rear in many more ways than this. Even disregarding the question of whether it's semantically appropriate, getting the planner to handle IS NULL this way would be a significant amount of work. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Index order ignored after `is null` in query
After some more investigation my wild guess would be that then nulls are involved in query postgresql wants to double check whatever they are really nulls in actual relation (maybe because of dead tuples). To do that it has to go and fetch pages from disk and the best way to do that is to use bitmap index. Sadly bitmaps tend to be not the best option when using limit in queries. Which would make sense, if it is really a need to synchronize index with relation... -- Best Regard, Artūras Lapinskas On Wed, Nov 05, 2014 at 10:42:43PM +0100, Artūras Lapinskas wrote: Hello, I am having some hard time understanding how postgresql handles null values. As much I understand null values are stored in b-tree as simple values (put as last or first depending on index). But it seems that there is something really specific about them as postgresql deliberately ignores obvious (I think...) optimizations concerning index order after using one of them in a query. As a simple example look at table below: arturas=# drop table if exists test; DROP TABLE arturas=# create table test ( arturas(# a int not null, arturas(# b int, arturas(# c int not null arturas(# ); CREATE TABLE After filling this table with random data (actual distribution of null's/real values seams not to matter): arturas=# insert into test (a, b, c) arturas-# select arturas-# case when random() < 0.5 then 1 else 2 end arturas-# , case when random() < 0.5 then null else 1 end arturas-# , case when random() < 0.5 then 1 else 2 end arturas-# from generate_series(1, 100, 1) as gen; INSERT 0 100 And creating index: arturas=# create index test_idx on test (a, b nulls first, c); CREATE INDEX We get fast queries with `order by` on c: arturas=# explain analyze verbose select * from test where a = 1 and b = 1 order by c limit 1; QUERY PLAN --- Limit (cost=0.42..0.53 rows=1 width=12) (actual time=0.052..0.052 rows=1 loops=1) Output: a, b, c -> Index Only Scan using test_idx on public.test (cost=0.42..25890.42 rows=251433 width=12) (actual time=0.051..0.051 rows=1 loops=1) Output: a, b, c Index Cond: ((test.a = 1) AND (test.b = 1)) Heap Fetches: 1 Total runtime: 0.084 ms (7 rows) But really slow ones if we search for null values of b: arturas=# explain analyze verbose select * from test where a = 1 and b is null order by c limit 1; QUERY PLAN - Limit (cost=15632.47..15632.47 rows=1 width=12) (actual time=138.127..138.127 rows=1 loops=1) Output: a, b, c -> Sort (cost=15632.47..16253.55 rows=248434 width=12) (actual time=138.127..138.127 rows=1 loops=1) Output: a, b, c Sort Key: test.c Sort Method: top-N heapsort Memory: 25kB -> Bitmap Heap Scan on public.test (cost=6378.87..14390.30 rows=248434 width=12) (actual time=47.083..88.986 rows=249243 loops=1) Output: a, b, c Recheck Cond: ((test.a = 1) AND (test.b IS NULL)) -> Bitmap Index Scan on test_idx (cost=0.00..6316.77 rows=248434 width=0) (actual time=46.015..46.015 rows=249243 loops=1) Index Cond: ((test.a = 1) AND (test.b IS NULL)) Total runtime: 138.200 ms (12 rows) Can someone please give some insight on this problem :) P.S. I am using `select version()` => PostgreSQL 9.3.5 on x86_64-unknown-linux-gnu, compiled by gcc (Debian 4.7.2-5) 4.7.2, 64-bit, compiled from source with no default configuration changes. -- Best Regard, Artūras Lapinskas -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
[PERFORM] Index order ignored after `is null` in query
Hello, I am having some hard time understanding how postgresql handles null values. As much I understand null values are stored in b-tree as simple values (put as last or first depending on index). But it seems that there is something really specific about them as postgresql deliberately ignores obvious (I think...) optimizations concerning index order after using one of them in a query. As a simple example look at table below: arturas=# drop table if exists test; DROP TABLE arturas=# create table test ( arturas(# a int not null, arturas(# b int, arturas(# c int not null arturas(# ); CREATE TABLE After filling this table with random data (actual distribution of null's/real values seams not to matter): arturas=# insert into test (a, b, c) arturas-# select arturas-# case when random() < 0.5 then 1 else 2 end arturas-# , case when random() < 0.5 then null else 1 end arturas-# , case when random() < 0.5 then 1 else 2 end arturas-# from generate_series(1, 100, 1) as gen; INSERT 0 100 And creating index: arturas=# create index test_idx on test (a, b nulls first, c); CREATE INDEX We get fast queries with `order by` on c: arturas=# explain analyze verbose select * from test where a = 1 and b = 1 order by c limit 1; QUERY PLAN --- Limit (cost=0.42..0.53 rows=1 width=12) (actual time=0.052..0.052 rows=1 loops=1) Output: a, b, c -> Index Only Scan using test_idx on public.test (cost=0.42..25890.42 rows=251433 width=12) (actual time=0.051..0.051 rows=1 loops=1) Output: a, b, c Index Cond: ((test.a = 1) AND (test.b = 1)) Heap Fetches: 1 Total runtime: 0.084 ms (7 rows) But really slow ones if we search for null values of b: arturas=# explain analyze verbose select * from test where a = 1 and b is null order by c limit 1; QUERY PLAN - Limit (cost=15632.47..15632.47 rows=1 width=12) (actual time=138.127..138.127 rows=1 loops=1) Output: a, b, c -> Sort (cost=15632.47..16253.55 rows=248434 width=12) (actual time=138.127..138.127 rows=1 loops=1) Output: a, b, c Sort Key: test.c Sort Method: top-N heapsort Memory: 25kB -> Bitmap Heap Scan on public.test (cost=6378.87..14390.30 rows=248434 width=12) (actual time=47.083..88.986 rows=249243 loops=1) Output: a, b, c Recheck Cond: ((test.a = 1) AND (test.b IS NULL)) -> Bitmap Index Scan on test_idx (cost=0.00..6316.77 rows=248434 width=0) (actual time=46.015..46.015 rows=249243 loops=1) Index Cond: ((test.a = 1) AND (test.b IS NULL)) Total runtime: 138.200 ms (12 rows) Can someone please give some insight on this problem :) P.S. I am using `select version()` => PostgreSQL 9.3.5 on x86_64-unknown-linux-gnu, compiled by gcc (Debian 4.7.2-5) 4.7.2, 64-bit, compiled from source with no default configuration changes. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
[PERFORM] Index order ignored after `is null` in query
Hello, I am having some hard time understanding how postgresql handles null values. As much I understand null values are stored in b-tree as simple values (put as last or first depending on index). But it seems that there is something really specific about them as postgresql deliberately ignores obvious (I think...) optimizations concerning index order after using one of them in a query. As a simple example look at table below: arturas=# drop table if exists test; DROP TABLE arturas=# create table test ( arturas(# a int not null, arturas(# b int, arturas(# c int not null arturas(# ); CREATE TABLE After filling this table with random data (actual distribution of null's/real values seams not to matter): arturas=# insert into test (a, b, c) arturas-# select arturas-# case when random() < 0.5 then 1 else 2 end arturas-# , case when random() < 0.5 then null else 1 end arturas-# , case when random() < 0.5 then 1 else 2 end arturas-# from generate_series(1, 100, 1) as gen; INSERT 0 100 And creating index: arturas=# create index test_idx on test (a, b nulls first, c); CREATE INDEX We get fast queries with `order by` on c: arturas=# explain analyze verbose select * from test where a = 1 and b = 1 order by c limit 1; QUERY PLAN --- Limit (cost=0.42..0.53 rows=1 width=12) (actual time=0.052..0.052 rows=1 loops=1) Output: a, b, c -> Index Only Scan using test_idx on public.test (cost=0.42..25890.42 rows=251433 width=12) (actual time=0.051..0.051 rows=1 loops=1) Output: a, b, c Index Cond: ((test.a = 1) AND (test.b = 1)) Heap Fetches: 1 Total runtime: 0.084 ms (7 rows) But really slow ones if we search for null values of b: arturas=# explain analyze verbose select * from test where a = 1 and b is null order by c limit 1; QUERY PLAN - Limit (cost=15632.47..15632.47 rows=1 width=12) (actual time=138.127..138.127 rows=1 loops=1) Output: a, b, c -> Sort (cost=15632.47..16253.55 rows=248434 width=12) (actual time=138.127..138.127 rows=1 loops=1) Output: a, b, c Sort Key: test.c Sort Method: top-N heapsort Memory: 25kB -> Bitmap Heap Scan on public.test (cost=6378.87..14390.30 rows=248434 width=12) (actual time=47.083..88.986 rows=249243 loops=1) Output: a, b, c Recheck Cond: ((test.a = 1) AND (test.b IS NULL)) -> Bitmap Index Scan on test_idx (cost=0.00..6316.77 rows=248434 width=0) (actual time=46.015..46.015 rows=249243 loops=1) Index Cond: ((test.a = 1) AND (test.b IS NULL)) Total runtime: 138.200 ms (12 rows) Can someone please give some insight on this problem :) P.S. I am using `select version()` => PostgreSQL 9.3.5 on x86_64-unknown-linux-gnu, compiled by gcc (Debian 4.7.2-5) 4.7.2, 64-bit, compiled from source with no default configuration changes. -- Best Regard, Artūras Lapinskas -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
