[HACKERS] WIP Patch: Precalculate stable functions - Mailing list pgsql-hackers

Hello everyone!

Now in Postgresql only immutable functions are precalculated; stable 
functions are calculated for every row so in fact they don't differ from 
volatile functions.

There's a proposal to precalculate stable and immutable functions (= 
calculate once for all output rows, but as many times as function is 
mentioned in query), if they don't return a set and their arguments are 
constants or recursively precalculated functions. The same for 
operators' functions, strict functions, tracking functions. It can be 
very effective, for example, there's a comparison for full text search 
in messages (Intel® Core™ i5-6500 CPU @ 3.20GHz × 4, RAM 8Gb):

Without precalculation:

EXPLAIN (ANALYZE TRUE, BUFFERS TRUE) SELECT COUNT(*) FROM messages WHERE 
body_tsvector @@ to_tsquery('postgres');                                                                QUERY 
PLAN

------------------------------------------------------------------------------------------------------
------------------------------------ Aggregate  (cost=18714.82..18714.83 rows=1 width=8) (actual 
time=2275.334..2275.334 rows=1 loops=1)   Buffers: shared hit=309234 read=184261   ->  Bitmap Heap Scan on messages
(cost=66.93..18702.34rows=4991 
 
width=0) (actual time=70.661..224
7.462 rows=151967 loops=1)         Recheck Cond: (body_tsvector @@ to_tsquery('postgres'::text))         Rows Removed
byIndex Recheck: 118531         Heap Blocks: exact=56726 lossy=33286         Buffers: shared hit=309234 read=184261
   ->  Bitmap Index Scan on message_body_idx  (cost=0.00..65.68 
 
rows=4991 width=0) (actual time=
54.599..54.599 rows=151967 loops=1)               Index Cond: (body_tsvector @@ 
to_tsquery('postgres'::text))               Buffers: shared hit=1 read=37 Planning time: 0.493 ms Execution time:
2276.412ms
 
(12 rows)

With precalculation:

EXPLAIN (ANALYZE TRUE, BUFFERS TRUE) SELECT COUNT(*) FROM messages WHERE 
body_tsvector @@ to_tsquery('postgres');                                                                  QUERY 
PLAN

------------------------------------------------------------------------------------------------------
---------------------------------------- Aggregate  (cost=192269.70..192269.71 rows=1 width=8) (actual 
time=1458.679..1458.680 rows=1 loops=1)   Buffers: shared hit=309234 read=184261   ->  Bitmap Heap Scan on messages
(cost=1445.68..191883.51
 
rows=154474 width=0) (actual time=70.069
..1433.999 rows=151967 loops=1)         Recheck Cond: (body_tsvector @@ to_tsquery('postgres'::text))         Rows
Removedby Index Recheck: 118531         Heap Blocks: exact=56726 lossy=33286         Buffers: shared hit=309234
read=184261        ->  Bitmap Index Scan on message_body_idx  (cost=0.00..1406.81 
 
rows=154474 width=0) (actual t
ime=56.149..56.149 rows=151967 loops=1)               Index Cond: (body_tsvector @@ 
to_tsquery('postgres'::text))               Buffers: shared hit=1 read=37 Planning time: 1.644 ms Execution time:
1459.836ms
 
(12 rows)

Patch is attached. It isn't done yet:
- changing documentation (partly because of next lines);
- precalculation of expressions IS DISTINCT FROM and NULLIF which use 
nonvolatile equality operators;
- precalculation of expressions "scalar op ANY/ALL (array)" which use 
nonvolatile operators;
- precalculation of row compare expressions which use nonvolatile 
operators.

-- 
Marina Polyakova
Postgres Professional: http://www.postgrespro.com
+7 926 92 00 265