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Re: MDEV-10306 Wrong results with combination of CONCAT, SUBSTR and CONVERT in subquery
On Mar 01, Oleksandr Byelkin wrote:
> > So, I see few possible approaches to this and other similar queries:
> > 1. We specify that no Item's val method can modify the buffer of the
> > arguments. That is, CONCAT will always have to copy. SUBSTR won't
> > need to copy, because it doesn't modify the buffer, it only returns a
> > pointer into it.
> > 2. May be #1 is not strict enough, and we'll need to disallow pointers
> > into the arguments' buffer too. Because, perhaps, args->val_str()
> > could realloc and then the pointer will become invalid.
> IMHO 2 is most realistic and safe. I can imagine many situation when one
> item val_* called many times and have no idea how it easy can be avoided
> without major refactoring (it is about #3 & #4).
> > 3. A different approach would be to disallow one item to appear twice in
> > an expression. No idea how to do that.
> > 4. A variand of #3, an item can appear many times, but it'll be only
> > evaluated once per row. That still needs #1, but #2 is unnecessary.
> > Opinions? Ideas?
> IMHO 2 is good idea (Actually I thought that now it is done like 2)
Well, sure. It's the bullet-proof one, but most expensive performance
wise, many items will need to start copying the result, basically on
every level of the expression tree the string will need to be copied.
So I'd prefer we find something more light-weight.
Chief Architect MariaDB