Re: POC: make mxidoff 64 bits

On Tue, 12 Nov 2024 at 02:31, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:

> The wraparound logic is still not correct.

Yep, my fault. I forget to reset segment counter if wraparound is happened.
Fixed.

When I try to select from a table after upgrade that contains
> post-wraparound multixids:
>
> TRAP: failed Assert("offset != 0"), File:
> "../src/backend/access/transam/multixact.c", Line: 1353, PID: 63386
>
The problem was in converting offset segments. The new_entry index should
also bypass the invalid offset (0) value. Fixed.

>
> On a different note, I'm surprised you're rewriting member segments from
> scratch, parsing all the individual member groups and writing them out
> again. There's no change to the members file format, except for the
> numbering of the files, so you could just copy the files under the new
> names without paying attention to the contents. It's not wrong to parse
> them in detail, but I'd assume that it would be simpler not to.
>
Yes, at the beginning I also thought that it would be possible to get by
with simple copying. But in case of wraparound, we must "bypass" invalid
zero offset value. See, old 32 bit offsets a wrapped at 2^32, thus 0 values
appears in multixact.c So, they must be handled. Bypass, in fact. When we
are switched to the 64-bit offsets, we have two options:
1). Bypass every ((uint32) offset == 0) value in multixact.c;
2). Convert members and bypass invalid value once.

The first options seem too weird for me. So, we have to repack members and
bypass invalid value.

All patches are for master(at)38c18710b37a2d

--
Best regards,
Maxim Orlov.