mirror of
https://github.com/postgres/postgres.git
synced 2025-09-09 13:09:39 +03:00
5543677ec9
When all of the query's DISTINCT pathkeys have been marked as redundant
due to EquivalenceClasses existing which contain constants, we can just
implement the DISTINCT operation on a query by just limiting the number of
returned rows to 1 instead of performing a Unique on all of the matching
(duplicate) rows.
This applies in cases such as:
SELECT DISTINCT col,col2 FROM tab WHERE col = 1 AND col2 = 10;
If there are any matching rows, then they must all be {1,10}. There's no
point in fetching all of those and running a Unique operator on them to
leave only a single row. Here we effectively just find the first row and
then stop. We are obviously unable to apply this optimization if either
the col = 1 or col2 = 10 were missing from the WHERE clause or if there
were any additional columns in the SELECT clause.
Such queries are probably not all that common, but detecting when we can
apply this optimization amounts to checking if the distinct_pathkeys are
NULL, which is very cheap indeed.
Nothing is done here to check if the query already has a LIMIT clause. If
it does then the plan may end up with 2 Limits nodes. There's no harm in
that and it's probably not worth the complexity to unify them into a
single Limit node.
Author: David Rowley
Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/CAApHDvqS0j8RUWRUSgCAXxOqnYjHUXmKwspRj4GzVfOO25ByHA@mail.gmail.com
Discussion: https://postgr.es/m/MEYPR01MB7101CD5DA0A07C9DE2B74850A4239@MEYPR01MB7101.ausprd01.prod.outlook.com
45 lines
1.4 KiB
SQL
45 lines
1.4 KiB
SQL
--
|
|
-- SELECT_DISTINCT_ON
|
|
--
|
|
|
|
SELECT DISTINCT ON (string4) string4, two, ten
|
|
FROM onek
|
|
ORDER BY string4 using <, two using >, ten using <;
|
|
|
|
-- this will fail due to conflict of ordering requirements
|
|
SELECT DISTINCT ON (string4, ten) string4, two, ten
|
|
FROM onek
|
|
ORDER BY string4 using <, two using <, ten using <;
|
|
|
|
SELECT DISTINCT ON (string4, ten) string4, ten, two
|
|
FROM onek
|
|
ORDER BY string4 using <, ten using >, two using <;
|
|
|
|
-- bug #5049: early 8.4.x chokes on volatile DISTINCT ON clauses
|
|
select distinct on (1) floor(random()) as r, f1 from int4_tbl order by 1,2;
|
|
|
|
--
|
|
-- Test the planner's ability to use a LIMIT 1 instead of a Unique node when
|
|
-- all of the distinct_pathkeys have been marked as redundant
|
|
--
|
|
|
|
-- Ensure we also get a LIMIT plan with DISTINCT ON
|
|
EXPLAIN (COSTS OFF)
|
|
SELECT DISTINCT ON (four) four,two
|
|
FROM tenk1 WHERE four = 0 ORDER BY 1;
|
|
|
|
-- and check the result of the above query is correct
|
|
SELECT DISTINCT ON (four) four,two
|
|
FROM tenk1 WHERE four = 0 ORDER BY 1;
|
|
|
|
-- Ensure a Sort -> Limit is used when the ORDER BY contains additional cols
|
|
EXPLAIN (COSTS OFF)
|
|
SELECT DISTINCT ON (four) four,two
|
|
FROM tenk1 WHERE four = 0 ORDER BY 1,2;
|
|
|
|
-- Same again but use a column that is indexed so that we get an index scan
|
|
-- then a limit
|
|
EXPLAIN (COSTS OFF)
|
|
SELECT DISTINCT ON (four) four,hundred
|
|
FROM tenk1 WHERE four = 0 ORDER BY 1,2;
|