Looking Forward to Postgres 19: Query Hints
pgedge.com57 points by jjgreen 3 days ago
57 points by jjgreen 3 days ago
> How many of us have toggled enable_seqscan to off to force an index scan? Or thrown an OFFSET 0 into a subquery to prevent the planner from flattening it?
enable_nestloop = off here.
For us, joining many complex views quickly trips the planner up, so I'm really glad to see this.
> They break on upgrades.
The irony is so does the planner. I've seen queries working perfectly fine in older PG's suddenly run away in newer versions. So hints will actually bring stability.
Its also worth reading the original post by Robert Haas (the author of pg_plan_advice) on motivation/design: https://rhaas.blogspot.com/2026/03/pgplanadvice-plan-stabili...
Also, I'll add my perspective: I think "EXPLAIN (PLAN_ADVICE)" is a key piece to making this a plan stability feature, not (just) a hinting feature. The extensibility/framework pg_plan_advice adds is a foundation, that over time will over time address the age-old "Postgres doesn't have hints" problem, even if the initial release doesn't check all the boxes yet, e.g. no way to use advice for adjusting row/join estimates.
To give an example on extensibility: Some people that I've spoken to are asking "but why is it not a comment-style hint". There are reasons why Postgres didn't go that way for this release (comment parsing in core is non-existent today, and comments don't work correctly e.g. for functions), but its easy to write an extension that sets up an advisor hook to parse comments: https://github.com/pganalyze/pg_advice_comment
I'm not an expert in database hints, but the syntax looks very readable and composable. That's great thing to have got right.
man, Tom Lane has hated query hints for literally decades
did he finally come around?
I don't think Tom's perspective has necessarily changed (and there is certainly concern from others that this could cause less reports on planner bugs), but Tom is pretty good about not standing in the way of others (i.e. Robert Haas in this case) trying to make things work, and being open to new perspectives.
I do know that one of the important criteria for getting this in was that a bad advice can't cause the planner to fail, and that's something that was explicitly included in the design of pg_plan_advice.