Re: HEAD seems to generate larger WAL regarding GIN index

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, Mar 17, 2014 at 10:54 AM, Heikki Linnakangas
> <hlinnakangas(at)vmware(dot)com> wrote:
>> Heap and B-tree WAL records also rely on PageAddItem etc. to reconstruct the
>> page, instead of making a physical copy of the modified parts. And
>> _bt_restore_page even inserts the items physically in different order than
>> the normal codepath does. So for good or bad, there is some precedence for
>> this.

> Yikes.

Yeah. I think it's arguably a bug that _bt_restore_page works like that,
even though we've not been burnt up to now.

>> The imminent danger I see is if we change the logic on how the items are
>> divided into posting lists, and end up in a situation where a master server
>> adds an item to a page, and it just fits, but with the compression logic the
>> standby version has, it cannot make it fit. As an escape hatch for that, we
>> could have the WAL replay code try the compression again, with a larger max.
>> posting list size, if it doesn't fit at first. And/or always leave something
>> like 10 bytes of free space on every data page to make up for small
>> differences in the logic.

> That scares the crap out of me.

Likewise. Saving some WAL space is *not* worth this kind of risk.

regards, tom lane