út 28. 4. 2020 v 18:17 odesílatel James Coleman <jtc...@gmail.com> napsal:
> 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. > can be nice if new implementation was not slower then older in all environments and context (including plpgsql expressions) Regards Pavel > James >
