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Re: [Commits] 13b5098: MDEV-9531: GROUP_CONCAT with ORDER BY inside takes a lot of memory while it's executed


Hi, Oleksandr!

On Sep 26, Oleksandr Byelkin wrote:
> >
> > A hackish workaround could be to adjust tree->elements_limit (in
> > Item_func_group_concat::add) after each insertion. But in this case
> > it would be simpler to limit the tree by size (in bytes) and adjust
> > tree size after each insertion. What do you think about it?
> I think that even for characters there is not direct correspondence
> between bytes and number of characters... so it is possible to make
> limit by bytes in case we have only strings  and put it as <length> *
> <maximum bytes per symbol for given charset> (here better to talk to
> Bar to ask if there is a pitfalls as difference in client/server/item
> charsets (I think should not be)).
> Also we have to take into account that we have key representation in
> the tree so probably + 1 bytes for null (I am not sure).
> It is worse problem (IMHO) is that it will mean constant
> allocating/freeing memory (with limit by number of elements tree is
> kind of freeze when it reach the limit). I am not sure if it is
> serious problem.
> I do not know more obstacles except above. And Limit by size is
> already present in the tree, but it just free all tree and start from
> beginning (so should be changed).

Yup. I thought about someting like that: besides TREE::memory_limit, we
add, say, TREE::memory_reset_to.

And when tree size reaches memory_limit, the tree is shrunk to
memory_reset_to. When memory_reset_to is 0, we get the old behavior,
full tree reset. If it's not 0, elements are removed one by one until
the tree is shrunk appropriately. This provides the backward-compatible
interface and solves the constant malloc/free issue that you've
mentioned above. I'd think memory_limit should be at least 2x


* removing elements one by one is not very fast, if memory_reset_to is
  much lower than memory_limit, it's faster to create a new tree and
  copy first N elements into it. I suppose with memory_limit being more
  than 3x memory_reset_to, new tree is already faster.
* element size is not the same as Item string value length (tree element
  is record image with varchar columns in it). To use correct strings
  value lengths, the upper code needs to correct tree allocated size
  after each insertion. And after each deletion (if elements will be
  deleted to reduce the tree size).

Chief Architect MariaDB
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