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Re: Unsplitting btree index leaf pages

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Martijn van Oosterhout <kleptog(at)svana(dot)org>, Simon Riggs <simon(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Unsplitting btree index leaf pages
Date: 2005-12-22 20:11:28
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> We already do something similar for page deletions.  Empty pages are not
> deleted right away, but they are marked with BTP_DEAD, and then deleted
> on a subsequent vacuum.  Or something like that, I don't remember the
> exact details.

Right, and the reason for that is exactly that there might be a
concurrent indexscan already "in flight" to the newly-dead page.
We must wait to recycle the page until we are certain no such scans

It doesn't matter whether a concurrent indexscan visits the dead
page or not, *because it's empty* and so there's nothing to miss.
So there's no race condition.  But if you try to move valid data
across pages then there is a race condition.

			regards, tom lane

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