Re: Evaluate arguments of correlated SubPlans in the referencing ExprState

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Evaluate arguments of correlated SubPlans in the referencing ExprState
Date: 2023-03-02 19:33:35
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Andres Freund <andres(at)anarazel(dot)de> writes:
> Around
> I suggested that we should evaluate the arguments of correlated SubPlans as
> part of the expression referencing the subplan.

> Here's a patch for that.

I looked through this, and there is one point that is making me really
uncomfortable. This bit is assuming that we can bind the address of
the es_param_exec_vals array right into the compiled expression:

+ ParamExecData *prm = &estate->es_param_exec_vals[paramid];
+ ExecInitExprRec(lfirst(pvar), state, &prm->value, &prm->isnull);

Even if that works today, it'd kill the ability to use the compiled
expression across more than one executor instance, which seems like
a pretty high price. Also, I think it probably fails already in
EvalPlanQual contexts, because EvalPlanQualStart allocates a separate
es_param_exec_vals array for EPQ execution.

I think we'd be better off inventing an EEOP_SET_PARAM_EXEC step type
that is essentially the inverse of EEOP_PARAM_EXEC/ExecEvalParamExec,
and then evaluating each parameter value into the expression's
scratch Datum/isnull fields and emitting SET_PARAM_EXEC to copy those
to the correct ParamExecData slot.

regards, tom lane

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