Getting an optimal plan on the first execution of a pl/pgsql function

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"

Thank you in advance,
Pedro Ivo