[SQL] (Mis)using the PostgreSQL planner to get estimated row counts

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[SQL] (Mis)using the PostgreSQL planner to get estimated row counts

Nick Johnson-11
I'm trying to write a PostgreSQL extension to estimate the number of
rows returned by a SELECT statement. Ideally, it'd be invoked along the
lines of "SELECT estimate_row_count('SELECT foo FROM bar INNER JOIN baz
ON (id) WHERE a=b');", and would be useful for estimating the number of
pages in a search result, for example.

I've got as far as figuring out how to get an estimated row count from a
Node object for a query (mostly by looking at the code for EXPLAIN), but
there I'm a bit mired - I can't figure out how to take a string
representing an SQL statement and parse it into a Node object I can feed
to the planner.

So, a couple of questions:
1) Can anyone suggest where I should look in the source for the
requisite functions for parsing an SQL string into a Node I can feed to
the planner so I can get an estimated row count?
2) Is this something that's reasonable to do in the first place? I'm not
sure if extensions are supposed to be allowed to delve into PostgreSQL's
internals this much.
3) Are there any other gotchas around this area? For example, there's a
lot going on with Snapshots and ActiveSnapshot that I really don't have
any idea about.

-Nick Johnson

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