Re: FIX : teach expression walker about RestrictInfo

Hi,

On 04/29/15 05:55, Tom Lane wrote:
> Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
>> On 04/28/15 21:50, Tom Lane wrote:
>>> RestrictInfo is not a general expression node and support for it has
>>> been deliberately omitted from expression_tree_walker(). So I think
>>> what you are proposing is a bad idea and probably a band-aid for some
>>> other bad idea.
>
>> That's not what I said, though. I said that calling pull_varnos() causes
>> the issue - apparently that does not happen in master, but I ran into
>> that when hacking on a patch.
>
> pull_varnos is not, and should not be, applied to a RestrictInfo; for one
> thing, it'd be redundant with work that was already done when creating the
> RestrictInfo (cf make_restrictinfo_internal). You've not shown the
> context of your problem very fully, but in general most code in the planner
> should be well aware of whether it is working with a RestrictInfo (or list
> of same) or a bare expression. I don't believe that it's a very good idea
> to obscure that distinction.

OK, let me explain the context a bit more. When working on the
multivariate statistics patch, I need to choose which stats to use for
estimating the clauses. I do that in clauselist_selectivity(), although
there might be other places where to do that.

Selecting the stats that best match the clauses is based on how well
they cover the vars in the clauses (and some other criteria, that is
mostly irrelevant here). So at some point I do call functions like
pull_varnos() and pull_varattnos() to get this information.

I do handle RestrictInfo nodes explicitly - for those nodes I can do get
the info from the node itself. But this does not work for the condition
I posted, because that contains nested RestrictInfo nodes. So I do call
pull_varnos() on a BoolExpr node, but in reality the node tree
representing the clauses

WHERE (a >= 10 AND a <= 20) OR (b >= 20 AND b <= 40)

looks like this:

BoolExpr [OR_EXPR]
BoolExpr [AND_EXPR]
RestrictInfo
OpExpr [Var >= Const]
RestrictInfo
OpExpr [Var <= Const]
BoolExpr [AND_EXPR]
RestrictInfo
OpExpr [Var >= Const]
RestrictInfo
OpExpr [Var <= Const]

so the pull_varnos() fails because it runs into RestrictInfo node while
walking the tree.

So I see three possibilities:

1) pull_varnos/pull_varattnos (and such functions, using the
expression walker internally) are not supposed to be called unless
you are sure there are no RestrictInfo nodes in the tree, but this
seems really awkward

2) expression walker is supposed to know about RestrictInfo nodes (as I
did in my patch), but you say this is not the case

3) pull_varnos_walker / pull_varattnos_walker are supposed to handle
RestrictInfo nodes explicitly (either by using the cached information
or by using RestrictInfo->clause in the next step)

regards

--
Tomas Vondra http://www.2ndQuadrant.com/
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