Re: [PATCH] Support for Array ELEMENT Foreign Keys

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Noah Misch <noah(at)leadboat(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org, Andrew Dunstan <andrew(at)dunslane(dot)net>, Marco Nenciarini <marco(dot)nenciarini(at)2ndquadrant(dot)it>
Subject: Re: [PATCH] Support for Array ELEMENT Foreign Keys
Date: 2012-10-24 16:06:35
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Noah Misch <noah(at)leadboat(dot)com> writes:
> For FKs, we currently document that "The referenced columns must be the
> columns of a non-deferrable unique or primary key constraint in the referenced
> table." Taking that literally, one might imagine that bare UNIQUE indexes do
> not qualify. However, transformFkeyCheckAttrs() does accept them, including
> indexes with non-default operator classes:

Indeed, and considerable sweat was spilled to make that happen. I'm
pretty unimpressed with any proposal that we should just blow that off
for array keys. Now, I concede that cross-type FKs are a corner case to
begin with, and we may well end up concluding that it's just too much
work to handle it for arrays because of the lack of infrastructure for
applying non-default comparison operators to arrays. But I don't want
that to happen just because we failed to even think about it.

However, I'm about to bounce this patch back for rework anyway, because
I've just noticed that it has fatal performance issues. If you issue
any UPDATE or DELETE against the PK table, you get a query like this
(shorn of some uninteresting syntactic details) for checking to see
if the RI constraint would be violated:

SELECT 1 FROM ONLY fktable x WHERE $1 = ANY (fkcol) FOR SHARE OF x;

It is impossible to implement this query except with a full-table
seqscan on the FK table. You can put a GIN index on the array fkcol,
but that won't help, because "something = ANY (indexedcol)" isn't an
indexable condition. I don't think we can ship a feature that's
unusable for anything except toy-sized tables, and that's what this is
right now.

One way we could consider making this GIN-indexable is to change it to

SELECT 1 FROM ONLY fktable x WHERE ARRAY[$1] <@ fkcol FOR SHARE OF x;

However, that puts us right back into the problem that we have no
control over the specific comparison semantics that <@ uses.

Or we could try to teach PG to make "something = ANY (indexedcol)"
indexable. That would likely be a pretty substantial amount of work
though. In particular, matching such a query to a GIN index would
require knowing whether the "=" operator corresponds to the idea of
equality embodied in the GIN opclass's key compare() method, and that
isn't information that's available from the current opclass API.

regards, tom lane

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