Hello,
I have found that explain on tables with many (hundreds) columns
are slow compare to nominal executions.
This can break application performances when using auto_explain or
pg_store_plans.
Here is my test case (with 500 columns, can be pushed to 1000 or 1600)
create table a();
DECLARE
i int;
BEGIN
for i in 1..500
loop
execute 'alter table a add column a'||i::text||' int';
end loop;
END
$$;
#\timing
#select a500 from a;
a500
------
(0 rows)
Time: 0,319 ms
#explain analyze select a500 from a;
QUERY PLAN
--------------------------------------------------------------------------------------------
Seq Scan on a (cost=0.00..10.40 rows=40 width=4) (actual time=0.010..0.010
rows=0 loops=1)
Planning time: 0.347 ms
Execution time: 0.047 ms
(3 rows)
Time: 4,290 ms
Here is a loop to try to understand where this comes from
DO
$$
DECLARE
i int;
j int;
BEGIN
for j in 1..100
loop
for i in 1..500
loop
execute 'explain select a'||i::text||' from a';
end loop;
end loop;
END
$$;
Using perf top, most of the cpu time seems to come from relutils.c
colname_is_unique:
59,54% libc-2.26.so [.] __GI___strcmp_ssse3
26,11% postgres [.] colname_is_unique.isra.2
1,46% postgres [.] AllocSetAlloc
1,43% postgres [.] SearchCatCache3
0,70% postgres [.] set_relation_column_names
0,56% libc-2.26.so [.] __strlen_avx2
select version();
PostgreSQL 11devel on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu
7.2.0-8ubuntu3) 7.2.0, 64-bit
Could this be improved ?
Regards
PAscal
--
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