Ibrar Ahmed <ibrar.ahmad@gmail.com> writes: > I give a quick look and I think in case whenever data is extracted from the > heap it shows all the columns. Therefore when columns are extracted from > the index only it shows the indexed column only.
This is operating as designed, and I don't think that the proposed patch is an improvement. The point of use_physical_tlist() is that returning all the columns is cheaper because it avoids a projection step. That's true for any case where we have to fetch the heap tuple, so IndexScan is included though IndexOnlyScan is not.
Now, that's something that was true a decade or more ago. There's been considerable discussion recently about cases where it's not true anymore, for example with columnar storage or FDWs, and so we ought to invent a way to prevent createplan.c from doing it when it would be counterproductive. But just summarily turning it off is not an improvement.
regards, tom lane
Hi,
In createplan.c, there is `change_plan_targetlist`
Plan *
change_plan_targetlist(Plan *subplan, List *tlist, bool tlist_parallel_safe)
But it doesn't have `Path` as parameter.
So I am not sure whether the check of non-returnable columns should be done in change_plan_targetlist().
bq. for example with columnar storage or FDWs,
Yeah. The above is the case where I want to optimize.
Cheers
Hi, Tom:
I was looking at the following comment in createplan.c :
* For table scans, rather than using the relation targetlist (which is * only those Vars actually needed by the query), we prefer to generate a * tlist containing all Vars in order. This will allow the executor to
* optimize away projection of the table tuples, if possible.
Maybe you can give me some background on the above decision.
