Re: COPY FREEZE and PD_ALL_VISIBLE

On 21 October 2015 at 13:31, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:

> On Tue, Oct 20, 2015 at 7:02 AM, Robert Haas <robertmhaas(at)gmail(dot)com>
> wrote:
>
>> On Sun, Oct 18, 2015 at 5:23 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>> > I'm planning on adding a todo item to have COPY FREEZE set
>> PD_ALL_VISIBLE.
>> > Or is there some reason this can't be done?
>> >
>> > Since the whole point of COPY FREEZE is to avoid needing to rewrite the
>> > entire table, it seems rather perverse that the first time the table is
>> > vacuumed, it needs to rewrite the entire table.
>>
>> *facepalm*
>>
>> I don't know how hard that is to implement, but +1 for trying to
>> figure out a way.
>>
>
>
> It turns out it was pretty easy to set PD_ALL_VISIBLE on the new pages,
> since the code in hio that requests the relation to be extended already has
> info on the tuple's intended freeze status.
>
> Then you just need to refrain from clearing PD_ALL_VISIBLE when that tuple
> is actually written into the page. Not only because clearing would defeat
> the purpose, but also because it will cause an error--apparently the
> incipient page is not yet in a state where visibilitymap_clear is willing
> to deal with it.
>
> With this patch, you get a table which has PD_ALL_VISIBLE set for all
> pages, but which doesn't have a _vm file.
>

Patch is simple enough. All usage looks safe, so I reckon this is good.

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. No need to
wait until EOXact.

--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services