Count ( DISTINCT ... ) is too slow [CORE214] #542
Description
Submitted by: skopalik (skopalik)
SFID: 669770#
Submitted By: skopalik
select distinct is about 10x to 100x faster then
select count(distinct ).
Tested version: FB 1.0.2.908 and FB 1.5.0.1761
test case:
create table a ( a int);
create procedure aset( cnt int) as
declare variable i int;
begin
i=0;
while(i<cnt)do begin
insert into a VALUES (:cnt/1000)
i=i+1;
end
end
execute procedure aset (10000)
PC: Notebokk Celeron 500/ 128 MB RAM
There are result captured from wisql:
select distinct * from a
PLAN SORT ((A NATURAL))
A
===========
10
Current memory = 0
Delta memory = 0
Max memory = 0
Elapsed time= 0.13 sec
Buffers = 75
Reads = 0
Writes 0
Fetches = 20072
select count (distinct a) from a
PLAN (A NATURAL)
COUNT
===========
1
Current memory = 0
Delta memory = 0
Max memory = 0
Elapsed time= 10.86 sec
Buffers = 75
Reads = 0
Writes 0
Fetches = 20071
Commits: 329fe9d 14ba585 FirebirdSQL/fbt-repository@3d35567 FirebirdSQL/fbt-repository@90e412b
====== Test Details ======
Multiple measures show that ratio in 2.5.5 and 3.0 is about 1.05 - 1.10 (this is how query "select count(*) from ( select distinct id from ... )" is SLOWER than "select count(distinct id) from ..."). In rare cases, when number of unique values is 100'000, this ratio was ~1.5.
Threshold was selected to be 1.8.