Re: MultiXacts & WAL

>There is no "regular shared locks" in postgres in that sense. Shared locks
>are only used for maintaining FK integrity. Or by manually issuing a
>SELECT FOR SHARE, but that's also for maintaining integrity. MVCC
>rules take care of the "plain reads". If you're not familiar with MVCC,
>it's explained in chapter 12 of the manual.
>
>The source code in heapam.c also mentions Point In Time Recovery to
>require logging the locks, though I'm not sure why.

Thanks for your explanations, now I can see what was confusing me.
The problem with is that we don't know beforehand if a transaction is a
distributed one or not.

Feel free to write a benchmark to see how much difference the logging
makes! If it's significant, I'm sure we can figure out ways to improve it.

Now that i finally see that multixacts are due only to explicit shared lock requests or to FKs, I tend to agree with tom's original doubts about the actual impact of the multixact related logging activities. Of course in practice such an impact would vary from application to application, so it may still make sense for some classes of workloads to avoid multixact logging, assuming they contain no distributed transactions and finding an hack to know beforehand whether a transaction is distributed or not... BTW, if i manage to find some free time to do some performance tests, i'll sure let you know!

Thanks again,

Paolo
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