On 10/08/2019 02:14, Peter Geoghegan wrote:
The easy part was removing SortTuple.tupindex itself -- it was fairly
natural to stash that in the slab allocation for each tape. I used the
aset.c trick of having a metadata "chunk" immediately prior to address
that represents the allocation proper -- we can just back up by a few
bytes from stup.tuple to find the place to stash the tape number
during merging. The worst thing about this change was that it makes a
tape slab allocation mandatory in cases that previously didn't have
any need for a stup.tuple allocation (e.g. datum tuplesorts of
pass-by-value types), though only during merging. Since we must always
store the tapenum when merging, we always need a slab buffer for each
tape when merging. This aspect wasn't so bad.
Hmm. Wouldn't it be more straightforward to have the extra tupindex
field at the end of the struct? Something like:
typedef struct
{
void *tuple; /* the tuple itself */
Datum datum1; /* value of first key column */
bool isnull1; /* is first key column NULL? */
} SortTuple;
typedef struct
{
SortTuple stuple;
int tupindex; /* see notes above */
} MergeTuple;
The initial sorting phase would deal with SortTuples, and the merge
phase would deal with MergeTuples. The same comparison routines work
with both.
The hard/ugly part was getting rid of the remaining "unnecessary"
SortTuple field, isnull1. This involved squeezing an extra bit out of
the stup.tuple pointer, by stealing the least-significant bit. This
was invasive in about the way you'd expect it to be. It wasn't awful,
but it also wasn't something I'd countenance pursuing without getting
a fairly noticeable benefit for users. (Actually, the code that I
wrote so far *is* pretty awful, but I could certainly clean it up some
more if I felt like it.)
I think that the rough patch that I came up with gives us an accurate
picture of what the benefits of having SortTuples that are only 16
bytes wide are. The benefits seem kind of underwhelming at this point.
For something like a "SELECT COUNT(distinct my_int4_col) FROM tab"
query, which uses the qsort_ssup() qsort specialization, we can easily
go from getting an external sort to getting an internal sort. We can
maybe end up sorting about 20% faster if things really work out for
the patch.
If you separate the NULLs from non-NULLs in a separate array, as was
discussed back in 2016, instead of stealing a bit, you can squeeze some
instructions out of the comparison routines, which might give some extra
speedup.
But in cases that users really care about, such as REINDEX,
the difference is in the noise. ISTM that this is simple not worth the
trouble at this time. These days, external sorts are often slightly
faster than internal sorts in practice, due to the fact that we can do
an on-the-fly merge with external sorts, so we could easily hurt
performance by making more memory available!
Yeah, that's a bit sad.
That makes me think: even when everything fits in memory, it might make
sense to divide the input into a few batches, qsort them individually,
and do an on-the-fly merge of the batches. I guess I'm essentially
suggesting that we should use merge instead of quicksort for the
in-memory case, too.
If we had the concept of in-memory batches, you could merge together
in-memory and external batches. That might be handy. For example, when
doing an external sort, instead of flushing the last run to disk before
you start merging, you could keep it in memory. That might be
significant in the cases where the input is only slightly too big to fit
in memory.
- Heikki
