Re: Performance Improvement by reducing WAL for Update Operation

On 2014-03-03 16:27:05 +0200, Heikki Linnakangas wrote:
> Thanks. I have to agree with Robert though that using the pglz encoding when
> we're just checking for a common prefix/suffix is a pretty crappy way of
> going about it [1].
> 
> As the patch stands, it includes the NULL bitmap when checking for a common
> prefix. That's probably not a good idea, because it defeats the prefix
> detection in a the common case that you update a field from NULL to not-NULL
> or vice versa.
> 
> Attached is a rewritten version, which does the prefix/suffix tests directly
> in heapam.c, and adds the prefix/suffix lengths directly as fields in the
> WAL record. If you could take one more look at this version, to check if
> I've missed anything.

Have you rerun the benchmarks? I'd guess the CPU overhead of this
version is lower than earlier versions, but seing it tested won't be a
bad idea.

> This ought to be tested with the new logical decoding stuff as it modified
> the WAL update record format which the logical decoding stuff also relies,
> but I don't know anything about that.

Hm, I think all it needs to do disable delta encoding if
need_tuple_data (which is dependent on wal_level=logical).

Greetings,

Andres Freund

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