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Re: MDEV-13790 UNHEX() of a somewhat complicated CONCAT() returns NULL
On Nov 03, Alexander Barkov wrote:
> Hello Sergei,
> Please review a patch for MDEV-13790.
> It removes the attempt to search the best suitable buffer
> and just returns the result in "str" passed to val_str().
> I'm quite sure this optimization gives nothing.
> In can be useful only under very rare conditions,
> but generates so many bugs.
There're bunch of tricks. What do they all do, could you list them?
One handles the case when an argument is a substring of the already
concatenated string - this is very weird and rare, I agree.
Other accumulates the result in the first argument's result - this
avoids one memcpy per row. Doesn't sound like much, but it's not rare,
so should be very easy to benchmark.
Yet another doubles the buffer size every time - this might be useful to
keep, it easily fits in your new code.
What are other tricks that concat was using?
Btw, one comment about your patch — please don't call current_thd in a
Chief Architect MariaDB