Re: On disable_cost

On Sat, May 4, 2024 at 12:57 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> There is also some weirdness around needing to force use of tidscan
> if we have WHERE CURRENT OF.  But perhaps a different hack could be
> used for that.

Yeah, figuring out what to do about this was the trickiest part of the
experimental patch that I wrote last week. The idea of the current
code is that cost_qual_eval_walker charges disable_cost for
CurrentOfExpr, but cost_tidscan then subtracts disable_cost if
tidquals contains a CurrentOfExpr, so that we effectively disable
everything except TID scan paths and, I think, also any TID scan paths
that don't use the CurrentOfExpr as a qual. I'm not entirely sure
whether the last can happen, but I imagine that it might be possible
if the cursor refers to a query that itself contains some other kind
of TID qual.

It's not very clear that this mechanism is actually 100% reliable,
because we know it's possible in general for the costs of two paths to
be different by more than disable_cost. Maybe that's not possible in
this specific context, though: I'm not sure.

The approach I took for my experimental patch was pretty grotty, and
probably not quite complete, but basically I defined the case where we
currently subtract out disable_cost as a "forced TID-scan". I passed
around a Boolean called forcedTidScan which gets set to true if we
discover that some plan is a forced TID-scan path, and then we discard
any other paths and then only add other forced TID-scan paths after
that point. There can be more than one, because of parameterization.

But I think that the right thing to do is probably to pull some of the
logic up out of create_tidscan_paths() and decide ONCE whether we're
in a forced TID-scan situation or not. If we are, then
set_plain_rel_pathlist() should arrange to create only forced TID-scan
paths; otherwise, it should proceed as it does now.

Maybe if I try to do that I'll find problems, but the current approach
seems backwards to me, like going to a restaurant and ordering one of
everything on the menu, then cancelling all of the orders except the
stuff you actually want.

-- 
Robert Haas
EDB: http://www.enterprisedb.com