Date: Wed, 5 Nov 2014 22:42:43 +0100
From: =?utf-8?Q?Art=C5=ABras?= Lapinskas <arturaslape@gmail.com>
To: postgres performance list <pgsql-performance@postgresql.org>
Subject: Index order ignored after `is null` in query
Message-ID: <20141105214243.GA546@public-docking-cx-1771.ethz.ch>
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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, 1000000, 1) as gen;
	INSERT 0 1000000

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


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