Re: Performance Improvement by reducing WAL for Update Operation

On 28 December 2012 11:27, Amit Kapila <amit.kapila@huawei.com> wrote:

>> * TOAST is not handled at all. No comments about it, nothing. Does
>> that mean it hasn't been considered? Or did we decide not to care in
>> this release?
>
>> Presumably that means we are comparing toast pointers
>> byte by byte to see if they are the same?
>
> Yes, currently this patch is doing byte by byte comparison for toast
> pointers. I shall add comment.
> In future, we can evaluate if further optimizations can be done.

Just a comment to say that the comparison takes place after TOASTed
columns have been removed. TOAST is already optimised for whole value
UPDATE anyway, so that is the right place to produce the delta.

It does make me think that we can further optimise TOAST by updating
only the parts of a toasted datum that have changed. That will be
useful for JSON and XML applications that change only a portion of
large documents.

-- Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services