From: Tom Lane <tgl@sss.pgh.pa.us>
To: Michael Paquier <michael@paquier.xyz>
cc: exclusion@gmail.com, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #18135: Incorrect memory access occurs when attaching a
 partition with an index
In-reply-to: <ZRoxY3X1tGZRZQ2A@paquier.xyz>
References: <18135-532f4a755e71e4d2@postgresql.org>
 <1043711.1695922299@sss.pgh.pa.us> <ZRi96-kUPFy0ILzb@paquier.xyz>
 <1614549.1696120777@sss.pgh.pa.us> <ZRoxY3X1tGZRZQ2A@paquier.xyz>
Comments: In-reply-to Michael Paquier <michael@paquier.xyz>
	message dated "Mon, 02 Oct 2023 11:56:35 +0900"
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Date: Sun, 01 Oct 2023 23:04:11 -0400
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Michael Paquier <michael@paquier.xyz> writes:
> FWIW, I was thinking about a case like that with 2 index attributes:
> info1->attr[0] = 0, info1->attr[1] = 1
> info2->attr[0] = 1, info2->attr[1] = 0
> With a mapping from info2 to info1 like that:
> attmap[0] = 1, attmap[1] = 0.

> The code before 9f71e10d6 would have accessed an incorrect pointer.
> The new code would return false, which would not be correct if the
> expressions stored in ii_Expressions for info1/attr0 and info2/attr1
> match?

I think "false" is correct.  Otherwise you're claiming that an
index on (expr, col1) is equivalent to an index on (col1, expr).
Maybe it is equivalent for some purposes, but I don't think this
function has any business assuming that it is so for all purposes.

Also, the pre-existing comment clearly intends that false is
the proper result here:

     * might differ!)  Note that this implies that index columns that are
     * expressions appear in the same positions.  We will next compare the
     * expressions themselves.

The code just failed to enforce that fully.

			regards, tom lane