On 04/16/2018 10:42 PM, Hackety Man wrote:
...
The first thing I did was to run some baseline tests using the basic
queries inside of the IF() checks found in each of the functions to
see how the query planner handled them. I ran the following two
queries.
EXPLAIN (ANALYZE, BUFFERS) SELECT COUNT(*) FROM zz_noidx1 WHERE
LOWER(text_distinct) = LOWER('Test5000001');
EXPLAIN (ANALYZE, BUFFERS) SELECT 1 FROM zz_noidx1 WHERE
LOWER(text_distinct) = LOWER('Test5000001');
Those are not the interesting plans, though. The EXISTS only cares about
the first row, so you should be looking at
EXPLAIN (ANALYZE, BUFFERS) SELECT 1 FROM zz_noidx1 WHERE
LOWER(text_distinct) = LOWER('Test5000001') LIMIT 1;
I moved on to test the other function with the following query...
EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM
zz_spx_ifcount_noidx('Test5000001');
and I got the following "auto_explain" results...
2018-04-16 14:58:34.134 EDT [12616] LOG: duration: 426.279 ms
plan:
Query Text: SELECT EXISTS (SELECT 1 FROM zz_noidx1 WHERE
LOWER(zz_noidx1.text_distinct) = LOWER(p_findme))
Result (cost=4.08..4.09 rows=1 width=1) (actual
time=426.274..426.274 rows=1 loops=1)
Buffers: shared read=5406
InitPlan 1 (returns $0)
-> Seq Scan on zz_noidx1 (cost=0.00..20406.00 rows=5000
width=0) (actual time=426.273..426.273 rows=0 loops=1)
Filter: (lower(text_distinct) = 'test5000001'::text)
Rows Removed by Filter: 1000000
Buffers: shared read=5406
2018-04-16 14:58:34.134 EDT [12616] CONTEXT: SQL statement
"SELECT EXISTS (SELECT 1 FROM zz_noidx1 WHERE
LOWER(zz_noidx1.text_distinct) = LOWER(p_findme))"
PL/pgSQL function zz_spx_ifexists_noidx(text) line 4 at IF
2018-04-16 14:58:34.134 EDT [12616] LOG: duration: 428.077 ms
plan:
Query Text: explain (analyze, buffers) select * from
zz_spx_ifexists_noidx('Test5000001')
Function Scan on zz_spx_ifexists_noidx (cost=0.25..0.26
rows=1 width=32) (actual time=428.076..428.076 rows=1 loops=1)
Buffers: shared hit=30 read=5438
Definitely not the execution time, or query plan, results I was
expecting. As we can see, no workers were employed here and my
guess was that this was the reason or the large execution time
difference between these 2 tests? 199 milliseconds versus 428
milliseconds, which is a big difference. Why are workers not being
employed here like they were when I tested the query found inside of
the IF() check in a standalone manner? But then I ran another test
and the results made even less sense to me.
The plan difference is due to not realizing the EXISTS essentially
implies LIMIT 1. Secondly, it expects about 5000 rows matching the
condition, uniformly spread through the table. But it apparently takes
much longer to find the first one, hence the increased duration.
How did you generate the data?
When I ran the above query the first 5 times after starting my
Postgres service, I got the same results each time (around 428
milliseconds), but when running the query 6 or more times, the
execution time jumps up to almost double that. Here are the
"auto_explain" results running this query a 6th time...
This is likely due to generating a generic plan after the fifth
execution. There seems to be only small difference in costs, though.
--"auto_explain" results after running the same query 6 or more
times.
2018-04-16 15:01:51.635 EDT [12616] LOG: duration: 761.847 ms
plan:
Query Text: SELECT EXISTS (SELECT 1 FROM zz_noidx1 WHERE
LOWER(zz_noidx1.text_distinct) = LOWER(p_findme))
Result (cost=4.58..4.59 rows=1 width=1) (actual
time=761.843..761.843 rows=1 loops=1)
Buffers: shared hit=160 read=5246
InitPlan 1 (returns $0)
-> Seq Scan on zz_noidx1 (cost=0.00..22906.00 rows=5000
width=0) (actual time=761.841..761.841 rows=0 loops=1)
Filter: (lower(text_distinct) = lower($1))
Rows Removed by Filter: 1000000
Buffers: shared hit=160 read=5246
2018-04-16 15:01:51.635 EDT [12616] CONTEXT: SQL statement
"SELECT EXISTS (SELECT 1 FROM zz_noidx1 WHERE
LOWER(zz_noidx1.text_distinct) = LOWER(p_findme))"
PL/pgSQL function zz_spx_ifexists_noidx(text) line 4 at IF
2018-04-16 15:01:51.635 EDT [12616] LOG: duration: 762.156 ms
plan:
Query Text: explain (analyze, buffers) select * from
zz_spx_ifexists_noidx('Test5000001')
Function Scan on zz_spx_ifexists_noidx (cost=0.25..0.26
rows=1 width=32) (actual time=762.154..762.155 rows=1 loops=1)
Buffers: shared hit=160 read=5246
As you can see, the execution time jumps up to about 762
milliseonds. I can see in the sequence scan node that the LOWER()
function shows up on the right side of the equal operator, whereas
in the first 5 runs of this test query the plan did not show this.
Why is this?
It doesn't really matter on which side it shows, it's more about a
generic plan built without knowledge of the parameter value.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
