Hi,
On 7/9/19 10:14 PM, Thomas Munro wrote:
Thomas, do you have any ideas for this ? I can see that MySQL did the
functionality in two change sets (base and function support), but like
you said we shouldn't paint ourselves into a corner.
I think amskip() could be augmented by later patches to take a
parameter 'skip mode' that can take values SKIP_FIRST, SKIP_LAST and
SKIP_FIRST | SKIP_LAST (meaning please give me both). What we have in
the current patch is SKIP_FIRST behaviour. Being able to choose
SKIP_FIRST or SKIP_LAST allows you do handle i, MAX(j) GROUP BY (i)
ORDER BY i (ie forward scan for the order, but wanting the highest key
for each distinct prefix), and being able to choose both allows for i,
MIN(j), MAX(j) GROUP BY i. Earlier I thought that this scheme that
allows you to ask for both might be incompatible with David's
suggestion of tracking UniqueKeys in paths, but now I see that it
isn't: it's just that SKIP_FIRST | SKIP_LAST would have no UniqueKeys
and therefore require a regular agg on top. I suspect that's OK: this
min/max transformation stuff is highly specialised and can have
whatever magic path-building logic is required in
preprocess_minmax_aggregates().
Ok, great.
Thanks for your feedback !
Best regards,
Jesper
