Re: POC: GROUP BY optimization

Hi,

Here's a rebased/improved version of the patch, with smaller parts
addressing various issues. There are seven parts:

0001 - main part, just rebased

0002 - replace the debug GUC options with a single GUC to disable the
optimization if needed

0003 - minor code cleanup, removal of unnecessary variable

0004 - various comment fixes (rewordings, typos, ...)

0005 - a minor code simplification, addressing FIXMEs from 0004

0006 - adds the new GUC to the docs

0007 - demonstrates plan changes with a disabled optimization

The first 6 parts should be squashed and committed at one, I only kept
them separate for clarity. The 0007 is merely a demonstration of the new
GUC and that it disables the optimization.

> Agree. Because it is a kind of automation we should allow user to switch
> it off in the case of problems or manual tuning.
> > Also, I looked through this patch. It has some minor problems:
> 1. Multiple typos in the patch comment.

I went through the comments and checked all of them for grammar mistakes
and typos using a word processor, so hopefully that should be OK. But
maybe there's still something wrong.

> 2. The term 'cardinality of a key' - may be replace with 'number of
> duplicates'?

No, cardinality means "number of distinct values", so "duplicates" would
be wrong. And I think "cardinality" is well established term, so I think
it's fine.

BTW I named the GUC enable_group_by_reordering, I wonder if it should be
named differently, e.g. enable_groupby_reordering? Opinions?

regards

--
Tomas Vondra
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