On 18/11/2024 22:22, Jeff Davis wrote:
On Mon, 2024-11-18 at 12:13 +0200, Heikki Linnakangas wrote:
Hmm, it would seem more straightforward to store it in the beginning,
i.e. have something like this:
struct {
void *additional;
MinimalTupleData mtup;
} ;
That was my first approach, but it requires an additional memcpy,
because ExecCopySlotMinimalTuple() does it's own palloc. I used
repalloc() because it will often have space at the end of the chunk
anyway, and not need to memcpy(). Maybe that's not significant but it
did seem detectable in some perf tests.
But perhaps we can go further and get rid of the "additional" pointer
and inline the pergroup data and the grouping key tuple into the same
palloc chunk? That would cut out a palloc and the 8 wasted bytes on the
pointer.
Sounds like a good idea. Needs some changes to the TupleTableSlotOps
interface to avoid the memcpy I presume.
Come to think of it, how important is it that we use MinimalTuple
here
at all? Some other representation could be faster to deal with in
TupleHashTableMatch() anyway.
What did you have in mind? That sounds like a good idea orthogonal to
reducing the bucket size.
Queries that have a only a small number of groups might might benefit
from storing a plain Datum/isnull array instead of a MinimalTuple. That
would take more memory when you have a lot of groups though.
Alternatively, MinimalTuple is not very "minimal", and perhaps we can
just make it better.
Yeah. It tries to be compatible with HeapTuple, but perhaps we should
give up on that and pack it more tightly instead.
0004: Removes the "status" field from TupleHashEntryData, using
firstTuple==NULL to mean "empty", otherwise "in use". Hack: need an
additional "special" pointer value to mean "input slot" now that
NULL
means "empty".
+1
For the FIRSTTUPLE_INPUTSLOT marker, do you think it's cleaner to use
what I did:
const static MinimalTuple FIRSTTUPLE_INPUTSLOT = (MinimalTuple) 0x1;
or something like:
static MinimalTupleData dummy = {0};
const static MinimalTuple FIRSTTUPLE_INPUTSLOT = &dummy;
?
I think I'd do "(MinimalTuple) 0x1" myself, but no strong opinion.
--
Heikki Linnakangas
Neon (https://neon.tech)
