Re: [HACKERS] Lazy hash table for XidInMVCCSnapshot (helps Zipfian a bit)

23.03.2018 17:59, Amit Kapila пишет:
> On Sat, Mar 10, 2018 at 7:41 AM, Yura Sokolov <funny(dot)falcon(at)gmail(dot)com> wrote:
>> 08.03.2018 03:42, Tomas Vondra пишет:
>>> One reason against building the hash table in GetSnapshotData is that
>>> we'd build it even when the snapshot is never queried. Or when it is
>>> queried, but we only need to check xmin/xmax.
>>
>> Thank you for analyze, Tomas.
>>
>> Stephen is right about bug in snapmgr.c
>> Attached version fixes bug, and also simplifies XidInXip a bit.
>>
>
> @@ -2167,8 +2175,7 @@ RestoreSnapshot(char *start_address)
> /* Copy SubXIDs, if present. */
> if (serialized_snapshot.subxcnt > 0)
> {
> - snapshot->subxip = ((TransactionId *) (snapshot + 1)) +
> - serialized_snapshot.xcnt;
> + snapshot->subxip = ((TransactionId *) (snapshot + 1)) + xcnt;
> memcpy(snapshot->subxip, serialized_xids + serialized_snapshot.xcnt,
> serialized_snapshot.subxcnt * sizeof(TransactionId));
> }
>
>
> It is not clear why you want to change this in RestoreSnapshot when
> nothing related is changed in SerializeSnapshot? Can you please add
> some comments to clarify it?
>

I didn't change serialized format. Therefore is no need to change
SerializeSnapshot.
But in-memory representation were changed, so RestoreSnapshot is changed.

With regards,
Sokolov Yura.