Re: why subplan is 10x faster then function?

1 октября 2017 г. 12:42:14 GMT+03:00, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> пишет:
>2017-09-30 23:23 GMT+02:00 Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>:
>
>> Hi
>>
>> I have some strange slow queries based on usage "view" functions
>>
>> one function looks like this:
>>
>> CREATE OR REPLACE FUNCTION
>ides_funcs.najdatsplt_cislo_exekuce(mid_najdatsplt
>> bigint)
>> RETURNS character varying
>> LANGUAGE sql
>> STABLE
>> AS $function$
>> select CISLOEXEKUCE
>> from najzalobpr MT, najvzallok A1,
>> NAJZALOBST A2, NAJZALOBCE A3 where
>> MT.ID_NAJVZALLOK= A1.ID_NAJVZALLOK AND
>> A1.ID_NAJZALOBST=A2.ID_NAJZALOBST AND
>> A2.ID_NAJZALOBCE= A3.ID_NAJZALOBCE AND
>> MT.ID_NAJDATSPLT = mID_NAJDATSPLT LIMIT 1;
>> $function$ cost 20
>> ;
>>
>> I know so using this kind of functions is not good idea - it is
>customer
>> old code generated from Oracle. I had idea about possible planner
>issues.
>> But this is a executor issue.
>>
>> when this function is evaluated as function, then execution needs
>about 46
>> sec
>>
>> -> Nested Loop Left Join (cost=0.71..780360.31 rows=589657
>> width=2700) (actual time=47796.588..47796.588 rows=0 loops=1)
>> -> Nested Loop (cost=0.29..492947.20 rows=589657
>width=2559)
>> (actual time=47796.587..47796.587 rows=0 loops=1)
>> -> Seq Scan on najdatsplt mt (cost=0.00..124359.24
>> rows=1106096 width=1013) (actual time=47796.587..47796.587 rows=0
>loops=1)
>> Filter:
>(najdatsplt_cislo_exekuce(id_najdatsplt) IS
>> NOT NULL)
>> Rows Removed by Filter: 1111654
>>
>> When I use correlated subquery, then
>>
>> -> Nested Loop (cost=0.29..19876820.11 rows=589657 width=2559)
>(actual
>> time=3404.154..3404.154 rows=0 loops=1)
>> -> Seq Scan on najdatsplt mt (cost=0.00..19508232.15 rows=1106096
>> width=1013) (actual time=3404.153..3404.153 rows=0 loops=1)
>> Filter: ((SubPlan 11) IS NOT NULL)
>> Rows Removed by Filter: 1111654
>> SubPlan 11
>> -> Limit (cost=1.10..17.49 rows=1 width=144) (actual
>> time=0.002..0.002 rows=0 loops=1111654)
>> -> Nested Loop (cost=1.10..17.49 rows=1 width=144)
>(actual
>> time=0.002..0.002 rows=0 loops=1111654)
>> -> Nested Loop (cost=0.83..17.02 rows=1
>width=8)
>> (actual time=0.002..0.002 rows=0 loops=1111654)
>> -> Nested Loop (cost=0.56..16.61 rows=1
>> width=8) (actual time=0.002..0.002 rows=0 loops=1111654)
>>
>> The execution plan is +/- same - the bottleneck is in function
>execution
>>
>> Tested with same result on 9.6, 10.
>>
>> Is known overhead of function execution?
>>
>>
>profile of slow execution looks like
>
>+ 24,71% 24,40% 48235 postmaster [.] SearchCatCache
>+ 14,25% 0,00% 0 postmaster [unknown] [.]
>0000000000000000
>+ 9,76% 9,65% 19071 postmaster [.]
>TupleDescInitEntry
>+ 3,91% 3,86% 7625 postmaster [.]
>ExecAssignScanProjectionInfoWithVarno
>+ 3,56% 3,52% 6955 postmaster [.] AllocSetAlloc
>+ 2,66% 2,63% 5193 postmaster [.]
>FunctionCall2Coll
>+ 2,65% 2,62% 5183 postmaster [.]
>ResourceArrayRemove
>+ 2,42% 2,39% 4719 postmaster [.]
>ExecTypeFromTLInternal
>+ 2,21% 2,19% 4321 postmaster [.]
>DirectFunctionCall1Coll
>+ 2,02% 2,00% 3961 postmaster [.]
>heap_getsysattr
>+ 1,85% 1,82% 3604 postmaster [.]
>exprTypmod
>+ 1,81% 1,79% 3540 postmaster [.]
>ResourceArrayAdd
>+ 1,68% 1,66% 3282 postmaster [.]
>hash_uint32
>+ 1,65% 1,63% 3214 postmaster [.]
>hash_search_with_hash_value
>+ 1,64% 1,62% 3208 postmaster [.]
>CatalogCacheComputeHashValue
>+ 1,28% 1,26% 2498 postmaster [.]
>MemoryContextAllocZeroAligned
>+ 1,25% 1,24% 2446 postmaster [.] palloc0
>
>Any ides why SearchCatCache is called too often?
>
>
>
>> Regards
>>
>> Pavel
>>

Looks like you've already collected profile with call-graph. So you can tell us where it were called from.

With regards,
--
Sokolov Yura aka funny_falcon