From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Simon Riggs <simon@2ndquadrant.com>,
 Amit Kapila <amit.kapila16@gmail.com>,
        Antonin Houska <ah@cybertec.at>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Possibly too stringent Assert() in b-tree code
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Mon, 19 Sep 2016 08:41:25 -0400"
Date: Thu, 22 Sep 2016 14:51:06 -0400
Message-ID: <8962.1474570266@sss.pgh.pa.us>
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Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Sep 19, 2016 at 7:07 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> I think you have a valid point.  It seems we don't need to write WAL
>> for reuse page (aka don't call _bt_log_reuse_page()), if the page is
>> new, as the only purpose of that log is to handle conflict based on
>> transaction id stored in special area which will be anyway zero.

> +1.

This is clearly an oversight in Simon's patch fafa374f2, which introduced
this code without any consideration for the possibility that the page
doesn't have a valid special area.  We could prevent the crash by
doing nothing if PageIsNew, a la

                if (_bt_page_recyclable(page))
                {
                    /*
                     * If we are generating WAL for Hot Standby then create a
                     * WAL record that will allow us to conflict with queries
                     * running on standby.
                     */
-                   if (XLogStandbyInfoActive() && RelationNeedsWAL(rel))
+                   if (XLogStandbyInfoActive() && RelationNeedsWAL(rel) &&
+                       !PageIsNew(page))
                    {
                        BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);

                        _bt_log_reuse_page(rel, blkno, opaque->btpo.xact);
                    }

                    /* Okay to use page.  Re-initialize and return it */

but I'm not very clear on whether this is a safe fix, mainly because
I don't understand what the reuse WAL record really accomplishes.
Maybe we need to instead generate a reuse record with some special
transaction ID indicating worst-case assumptions?

			regards, tom lane


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