Re: Why performance improvement on converting subselect - Mailing list pgsql-performance

Rajesh Kumar Mallah <mallah@trade-india.com> writes: 

explain analyze SELECT company_id , (SELECT edition FROM ONLY 
public.branding_master b WHERE old_company_id = a.company_id OR company_id = 
a.company_id  ORDER BY b.company_id DESC LIMIT 1) from public.branding_master
a   limit 50;
Total runtime: 19429.76 msec   

CREATE FUNCTION most_recent_edition (integer) returns integer AS 'SELECT 
edition::integer FROM ONLY public.branding_master b WHERE old_company_id = $1
OR company_id = $1  ORDER BY b.company_id DESC LIMIT 1 ' language 'sql';   

tradein_clients=# explain analyze SELECT company_id , 
most_recent_edition(company_id) from public.branding_master limit 50;
Total runtime: 3969.52 msec   

Odd.  Apparently the planner is picking a better plan in the function
context than in the subselect context --- which is strange since it
ought to have less information.

AFAIK the only way to see the plan generated for a SQL function's query
is like this:

regression=# create function foo(int) returns int as
regression-# 'select unique1 from tenk1 where unique1 = $1' language sql;
CREATE FUNCTION
regression=# set debug_print_plan TO 1;
SET
regression=# set client_min_messages TO debug;
SET
regression=# select foo(55);
DEBUG:  plan:
DETAIL:  {RESULT :startup_cost 0.00 :total_cost 0.01 :plan_rows 1 :plan_width 0
:targetlist ({TARGETENTRY :resdom {RESDOM :resno 1 :restype 23 :restypmod -1
:resname foo :ressortgroupref 0 :resorigtbl 0 :resorigcol 0 :resjunk false}
:expr {FUNCEXPR :funcid 706101 :funcresulttype 23 :funcretset false... (etc etc)

Would you do that and send it along?  I'm curious ...

tradein_clients=# explain analyze SELECT company_id ,  data_bank.most_recent_edition(company_id) from public.branding_master limit 50;

--------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..3.57 rows=50 width=4) (actual time=149.52..2179.49 rows=50 loops=1)
   ->  Seq Scan on branding_master  (cost=0.00..6626.52 rows=92752 width=4) (actual time=149.51..2179.30 rows=51 loops=1)
tradein_clients=#
tradein_clients=#
tradein_clients=# explain analyze SELECT company_id ,  data_bank.most_recent_edition(company_id) from public.branding_master limit 50;
DEBUG:  StartTransactionCommand
LOG:  plan:
{ LIMIT :startup_cost 0.00 :total_cost 185.65 :rows 1 :width 6 :qptargetlist
({ TARGETENTRY :resdom { RESDOM :resno 1 :restype 23 :restypmod -1 :resname
edition :reskey 0 :reskeyop 0 :ressortgroupref 0 :resjunk false } :expr { EXPR
:typeOid 23  :opType func :oper { FUNC :funcid 313 :funcresulttype 23
:funcretset false :funcformat 1 } :args ({ VAR :varno 1 :varattno 31 :vartype
21 :vartypmod -1  :varlevelsup 0 :varnoold 1 :varoattno 31})}} { TARGETENTRY
:resdom { RESDOM :resno 2 :restype 23 :restypmod -1 :resname company_id
:reskey 0 :reskeyop 0 :ressortgroupref 1 :resjunk true } :expr { VAR :varno 1
:varattno 1 :vartype 23 :vartypmod -1  :varlevelsup 0 :varnoold 1 :varoattno
1}}) :qpqual <> :lefttree { INDEXSCAN :startup_cost 0.00 :total_cost 24302.69
:rows 131 :width 6 :qptargetlist ({ TARGETENTRY :resdom { RESDOM :resno 1
:restype 23 :restypmod -1 :resname edition :reskey 0 :reskeyop 0
:ressortgroupref 0 :resjunk false } :expr { EXPR :typeOid 23  :opType func
:oper { FUNC :funcid 313 :funcresulttype 23 :funcretset false :funcformat 1 }
:args ({ VAR :varno 1 :varattno 31 :vartype 21 :vartypmod -1  :varlevelsup 0
:varnoold 1 :varoattno 31})}} { TARGETENTRY :resdom { RESDOM :resno 2 :restype
23 :restypmod -1 :resname company_id :reskey 0 :reskeyop 0 :ressortgroupref 1
:resjunk true } :expr { VAR :varno 1 :varattno 1 :vartype 23 :vartypmod -1
:varlevelsup 0 :varnoold 1 :varoattno 1}}) :qpqual ({ EXPR :typeOid 16
:opType or :oper <> :args ({ EXPR :typeOid 16  :opType op :oper { OPER :opno
96 :opid 65 :opresulttype 16 :opretset false } :args ({ VAR :varno 1 :varattno
19 :vartype 23 :vartypmod -1  :varlevelsup 0 :varnoold 1 :varoattno 19} {
PARAM :paramkind 12 :paramid 1 :paramname \<unnamed> :paramtype 23 })} { EXPR
:typeOid 16  :opType op :oper { OPER :opno 96 :opid 65 :opresulttype 16
:opretset false } :args ({ VAR :varno 1 :varattno 1 :vartype 23 :vartypmod -1
:varlevelsup 0 :varnoold 1 :varoattno 1} { PARAM :paramkind 12 :paramid 1
:paramname \<unnamed> :paramtype 23 })})}) :lefttree <> :righttree <> :extprm
() :locprm () :initplan <> :nprm 0  :scanrelid 1 :indxid ( 310742439)
:indxqual (<>) :indxqualorig (<>) :indxorderdir -1 } :righttree <> :extprm ()
:locprm () :initplan <> :nprm 0  :limitOffset <> :limitCount { CONST
:consttype 23 :constlen 4 :constbyval true :constisnull false :constvalue  4 [
1 0 0 0 ] }}

DEBUG:  CommitTransactionCommand

But i feel it can be lot more faster , can anyone suggest me something
to try.   

Create an index on old_company_id, perhaps.

		regards, tom lane

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