Re: Getting an optimal plan on the first execution of a pl/pgsql function - Mailing list pgsql-performance

I have a really busy function that I need to optimize the best way I can. This function is just a nested select statement that is requested several times a sec by a legacy application. I'm running a PostgreSQL 9.4 on a CentOS 6;

The indexes are in place but I've noticed that it is only used after the first execution of the function.

I think that the problem is that Postgres isn't getting the best execution plan at first because of a parameter that it is highly exclusive in the majority of the cases, but it can be not as good sometimes. We can't change the way we call the function to a plain sql statement or a view because we can't change the application code itself.

When I test with EXPLAIN ANALYZE after the first execution, the query runs really fast but the aplication sessions call the function only once and then are terminated. I need that the first execution use the actual optimized plan. 

We tried messing around with the connector driver that manage the connection pooling to issue a DISCARD TEMP instead of DISCARD ALL, so it could keep the cached plan of the sessions and the performance improved a lot, but I don't want to do that in a production environment.

I've tried to change the language to a sql function but it didn't help as the execution time didn't drop after the first execution.

I've tried to add the "SET LOCAL join_collapse_limit = 1" too but it appears it doesn't work inside a function;

Here is the function code:

CREATE OR REPLACE FUNCTION public.ap_keepalive_geteqpid_veiid(

    IN tcbserie bigint,

    IN protocolo integer)

  RETURNS TABLE(eqpid integer, veiid integer, tcbid integer, veiplaca character varying, veiproprietariocliid integer, tcbtppid integer, tcbversao character, veirpmparametro double precision, tcbconfiguracao bigint, tcbevtconfig integer, veibitsalertas integer, sluid integer, harid integer) AS

$BODY$

BEGIN

RETURN QUERY

SELECT teqp.eqpID, 

teqp.eqpveiID AS veiID, 

tcb.tcbID, 

tvei.veiPlaca, 

tvei.veiProprietariocliID, 

tcb.tcbtppID, 

tcb.tcbVersao,

tvei.veiRPMParametro, 

COALESCE(COALESCE(NULLIF(tcb.tcbConfiguracao, 0), tcc.clcConfiguracaoBitsVeic), 0) AS tcbConfiguracao,

COALESCE(tcb.tcbevtConfig, 0) AS tcbevtConfig,

COALESCE(tvei.veiBitsAlertas, 0) AS veiBitsAlertas,

COALESCE(tvei.veisluID, 0) AS sluID,

COALESCE(tcb.tcbharID, 0) AS harID

FROM TabComputadorBordo tcb

INNER JOIN TabEquipamento teqp ON teqp.eqptcbID = tcb.tcbID

INNER JOIN TabPacoteProduto tpp ON teqp.eqptppID = tpp.tppID

INNER JOIN TabVeiculos tvei ON teqp.eqpveiID = tvei.veiID

LEFT JOIN TabCliente tcli ON tcli.cliid = tvei.veiProprietariocliID

LEFT JOIN TabClienteConfig tcc ON tcc.clcCliID = tcli.cliID

WHERE   tcb.tcbserie = $1

AND teqp.eqpAtivo = 1

AND tpp.tppIDProtocolo = $2

AND tvei.veiBloqueioSinal = 0;

END

$BODY$

  LANGUAGE plpgsql VOLATILE

  COST 10000

  ROWS 1;

Execution plan in the first execution:

"Function Scan on ap_keepalive_geteqpid_veiid  (cost=0.25..0.26 rows=1 width=116) (actual time=3.268..3.268 rows=1 loops=1)"

"Planning time: 0.032 ms"

"Execution time: 3.288 ms"

Second execution:

"Function Scan on ap_keepalive_geteqpid_veiid  (cost=0.25..0.26 rows=1 width=116) (actual time=0.401..0.402 rows=1 loops=1)"

"Planning time: 0.058 ms"

"Execution time: 0.423 ms"