An Idea for planner hints

From: "Florian G(dot) Pflug" <fgp(at)phlo(dot)org>
To: pgsql-hackers(at)postgresql(dot)org
Subject: An Idea for planner hints
Date: 2006-08-08 11:55:35
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Since the discussion about how to force a specific plan has
come up, I though I'd post an idea I had for this a while ago.
It's not reall well though out yet, but anyway.

When the topic of optimizer hints comes up, people often suggest
that there should be a way to force postgres to use a certain
index, or do joins in a certain order. AFAIK, this mimics what
oracle does - you can put comments into your query that specify
what index to use. This approach has two major drawbacks
.) Plans that seem good now might not seem that good a few months
later - your data might have changed, and other execution plans
might fit better now
.) You have to change all your queries to make use of features
in new postgres versions, like bitmap scans.

My experience with the postgres optimizer is that it usually performs
great - and if it doesn't, that always boiled down to two problems
(at least for me)
.) The query is autogenerated, and includes complex, and highly inter-
dependent where (or join) conditions. This leads to wrong estimates
of where selectivity, and thus to bad plans.
.) There are correlations between columns and/or tables that postgres
doesn't know about (and has no chance of knowing about). Again, this
leads to vastly wrong estimates of row counts, and to bad plans.

I think that those bad estimates of the selectivity of where-clauses
(or on-clauses for joins) is where postgres could use hints.

Image a query like "select ... from t1 join t2 on t1.t2_id = and
<expr>". Lets say that "<expr>" is true for only 1% of the rows in t2 -
but those are exactly the rows that have matching rows in t1.

Postgres would probably guess that this join will produce about 1/100
of the rows that t1 has - but I _know_ that it will produce 100 (!)
times more rows.

Now, I'd like to hand that information to postgres. I wouldn't want
to force any particular access method or join order, but rather I'd
just tell it "hey, this expression has selectivity 1 in this context,
not 0.01 as you might think".

Could that work?

greetings, Florian Pflug


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