Re: Eager aggregation, take 3

I've run TPC-DS again to compare planning times with and without eager
aggregation. Out of 99 queries, only one query (query 64) shows a
noticeable increase in planning time. This query performs inner joins
across 38 tables. This is a very large search space. (I'm talking
about the standard join search method, not the GEQO.)

If my math doesn't fail me, the maximum number of different join
orders when joining n tables is: Catalan(n − 1) x n!. For n = 38,
this number is astronomically large. In practice, query 64 joins 19
tables twice (due to a CTE), which still results in about 3.4E28
different join orders.

Of course, in practice, with the help of join_collapse_limit and other
heuristics, the effective search space is reduced a lot, but even
then, it remains very large. Given this, I'm not too surprised that
query 64 shows an increase in planning time when eager aggregation is
applied -- exploring the best join order in such a space is inherently
expensive.

That said, I've identified a few performance hotspots that can be
optimized to help reduce planning time:

1) the exprs_known_equal() call in get_expression_sortgroupref(),
which is used to check if a given expression is known equal to a
grouping expression due to ECs. We can optimize this by storing the
EC of each grouping expression, and then get_expression_sortgroupref()
would only need to search the relevant EC, rather than scanning all of
them.

2) the estimate_num_groups() call in create_rel_agg_info(). We can
optimize this by avoiding unnecessary calls to estimate_num_groups()
where possible.

Attached is an updated version of the patch with these optimizations
applied. With this patch, the planning times for query 64, with and
without eager aggregation, are:

-- with eager aggregation
Planning Time: 9432.042 ms
-- without eager aggregation
Planning Time: 7196.999 ms

I think the increase in planning time is acceptable given the large
search space involved, though I may be biased.

- Richard