Re: COPY FREEZE and PD_ALL_VISIBLE - Mailing list pgsql-hackers

On Fri, Oct 23, 2015 at 6:29 AM, Simon Riggs <simon@2ndquadrant.com> wrote:

On 21 October 2015 at 13:31, Jeff Janes <jeff.janes@gmail.com> wrote:

Index-only scans will visit the heap for each tuple until the first VACUUM is done.

The first vacuum will read the entire table, but not need to write it anymore.  And will create the _vm file.

I think we really want to create _vm file as well as set PD_ALL_VISIBLE, but I don't know the best way to do that.  Set a flag somewhere and then create it in bulk at the end of the transaction?  Set it bit by bit as the pages are extended and initialized?

Easy enough to do it at the end of the COPY FREEZE in one step.

Here, we might want to consider that setting bit in visibility map

will generate WAL log whereas Copy Freeze otherwise skip WAL

when wal_level is less than archive.  This can lead to extra disk

writes which can slow down Copy Freeze, but OTOH that might

be acceptable.