On Tue, Apr 28, 2020 at 11:18 AM Pavel Stehule <pavel.steh...@gmail.com> wrote: > > > > út 28. 4. 2020 v 16:48 odesílatel Tomas Vondra <tomas.von...@2ndquadrant.com> > napsal: >> >> On Tue, Apr 28, 2020 at 03:43:43PM +0200, Pavel Stehule wrote: >> >út 28. 4. 2020 v 15:26 odesílatel Tomas Vondra >> ><tomas.von...@2ndquadrant.com> >> >napsal: >> > >> >> ... >> >> >> >> >I'm not so concerned about this in any query where we have a real FROM >> >> >clause because even if we end up with only one row, the relative >> >> >penalty is low, and the potential gain is very high. But simple >> >> >expressions in pl/pgsql, for example, are a case where we can know for >> >> >certain (correct me if I've wrong on this) that we'll only execute the >> >> >expression once, which means there's probably always a penalty for >> >> >choosing the implementation with setup costs over the default linear >> >> >scan through the array. >> >> > >> >> >> >> What do you mean by "simple expressions"? I'm not plpgsql expert and I >> >> see it mostly as a way to glue together SQL queries, but yeah - if we >> >> know a given ScalarArrayOpExpr will only be executed once, then we can >> >> disable this optimization for now. >> >> >> > >> >a := a + 1 >> > >> >is translated to >> > >> >SELECT $1 + 1 and save result to var a >> > >> >The queries like this "SELECT $1 + 1" are simple expressions. They are >> >evaluated just on executor level - it skip SPI >> > >> >the simple expression has not FROM clause, and have to return just one row. >> >I am not sure if it is required, it has to return just one column.
Yes, this is what I meant by simple expressions. >> >I am not sure if executor knows so expression is executed as simply >> >expressions. But probably it can be deduced from context >> > >> >> Not sure. The executor state is created by exec_eval_simple_expr, which >> calls ExecInitExprWithParams (and it's the only caller). And that in >> turn is the only place that leaves (state->parent == NULL). So maybe >> that's a way to identify simple (standalone) expressions? Otherwise we >> might add a new EEO_FLAG_* to identify these expressions explicitly. I'll look into doing one of these. >> I wonder if it would be possible to identify cases when the expression >> is executed in a loop, e.g. like this: >> >> FOR i IN 1..1000 LOOP >> x := y IN (1, 2, ..., 999); >> END LOOP; >> >> in which case we only build the ScalarArrayOpExpr once, so maybe we >> could keep the hash table for all executions. But maybe that's not >> possible or maybe it's pointless for other reasons. It sure looks a bit >> like trying to build a query engine from FOR loop. > > > Theoretically it is possible, not now. But I don't think so it is necessary. > I cannot to remember this pattern in any plpgsql code and I never seen any > request on this feature. > > I don't think so this is task for plpgsql engine. Maybe for JIT sometimes. Agreed. I'd thought about this kind of scenario when I brought this up, but I think solving it would the responsibility of the pg/pgsql compiler rather than the expression evaluation code, because it'd have to recognize the situation and setup a shared expression evaluation context to be reused each time through the loop. James
