Wrong selection of driving table for DELETE FROM T_HUGE WHERE EXISTS(SELECT *
FROM T_TINY ...)
----------------------------------------------------------------------------------------------
Key: CORE-3518
URL: http://tracker.firebirdsql.org/browse/CORE-3518
Project: Firebird Core
Issue Type: Bug
Affects Versions: 2.5.0
Reporter: Pavel Zotov
Suppose we have two tables: T_HUGE of 1'000'000 rows and T_TINY of only 100
rows:
recreate table t_huge(id int primary key using index t_huge_pk);
recreate table t_tiny(id int primary key using index t_tiny_pk);
commit;
set term ^;
execute block as
declare n int = 1000000;
declare n0 int;
declare q int = 100;
begin
n0=n;
while (n>0) do insert into t_huge values(:n) returning :n-1 into :n;
while (q>0) do insert into t_tiny values(rand()*:n0) returning :q-1 into :q;
end
^set term ;^
commit;
set statistics index t_huge_pk;
set statistics index t_tiny_pk;
commit;
If we have to delete all such records from T_HUGE that present also in T_TINY
(i.e. they have equal IDs) we could use such statement:
delete from t_huge t where exists(select * from t_tiny s where t.id=s.id)
But this variant has poor plan:
PLAN (S INDEX (T_TINY_PK))
PLAN (T NATURAL)
And also it has bad perfomance data:
100 record(s) was(were) deleted from T_HUGE
Execute time = 6s 94ms
...
Memory buffers = 1 024
Reads from disk to cache = 12 351
Writes from cache to disk = 89
Fetches from cache = 4 025 225
Table T_HUGE is *fully* scanned (NIR = 1000'000). Optimizer ignores cardinality
of T_TINY and selects T_HUGE as driving table. I thing this is obvious mistake.
Compare with another way, non-SQL:
execute block as
declare v_id type of column t_tiny.id;
begin
for
select id
from t_tiny
into v_id
do
delete from t_huge where id=:v_id;
end
Perfomance data:
Execute time = 125ms
...
Memory buffers = 1 024
Reads from disk to cache = 212
Writes from cache to disk = 0
Fetches from cache = 833
In this case we have only 100 NIRs for T_TINY and 100 IRs for T_HUGE.
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