Re: B-Tree support function number 3 (strxfrm() optimization) - Mailing list pgsql-hackers

On Thu, Jul 31, 2014 at 1:12 PM, Peter Geoghegan <pg@heroku.com> wrote:
> Abbreviated it is.

Attached revision uses new terminology. I have abandoned configure
tests entirely, preferring to explicitly discriminate against Mac OS X
(Darwin) as a platform with a known odd locale implementation, just
like pg_get_encoding_from_locale() does. Since Robert did not give a
reason for discussing the basic fmgr-elision patch despite having
already discussed it a couple of years ago, I have not split the patch
into two (if I did, the first patch would be virtually identical to
Robert's 2012 patch). I hope that's okay. I am willing to revisit this
question if Robert feels that something is uncertain about the costs
and benefits of a basic fmgr-eliding sort support for text.

There are still some open items:

* I have left the licensing question unresolved. There is plenty we
can do about this, and if necessary we can ask the original author to
changed his license from the MIT license to the PostgreSQL license.
AFAICT, the BSD license is *more* restrictive than the MIT license,
and the PostgreSQL license is identical. The wording is almost
identical. I don't see the concern. If the PostgreSQL license had the
“non-endorsement clause” of the New BSD license, or alternatively if
the MIT license had a similar clause that the PostgreSQL licensed
lacked, that would be a substantive and appreciable difference. That
isn't the case.

* I have not made aggregates use the optimization where they currently
accidentally don't due to using datum tuplesort. I can artificially
force them to use heap tuplesort where that's likely to help [1].
Let's defer this question until we have an abort algorithm that seems
reasonable. There is a FIXME item.

Improvements in full:

* New terminology ("Abbreviated keys").

* Better explanations for why we don't use the additional optimization
of abbreviated keys where we're using sort support, in analyze.c and
nodeMergeAppend.c.

* Better handling of NULLs.

* Never use the optimization with bounded heap sorts.

* Better abort strategy, that centers on the idea of measuring full
key cardinality, and abbreviated key cardinality, and weighing how
good a proxy the former is for the latter. This is heavily weighed
when deciding if we should abort normalization as tuples are copied.
Exact details are explained within bttext_abort(). As already
mentioned, this can allow us to use the optimization when we're
sorting a million tuples with only five distinct values. This will
still win decisively, but it's obviously impossible to make work while
only considering abbreviated key cardinality. Determining cardinality
of both abbreviated keys and full keys appears to have very low
overhead, and is exactly what we ought to care about, so that's what
we do. While there is still some magic to the algorithm's inputs, my
sense is that I'm much closer to characterizing the break-even point
than before.


I also attach a second patch, which adds additional debug
instrumentation, and is intended to be applied on top of the real
patch to help with review. Run Postgres with DEBUG1 output when it's
applied. With the patch applied, the setting backslash_quote also
controls whether or not the optimization is used. So with the debug
instrumentation patch applied:

"backslash_quote = on" - use optimization, but never abort

"backslash_quote = off" - Never use optimization - set up shim (just
like the win32 UTF-8 case). Equivalent to master's behavior.

"backslash_quote = safe_encoding" - Use optimization, but actually
abort if it doesn't work out, the behavior that is always seen without
instrumentation. This is useful for testing the overhead of
considering the optimization in cases where it didn't work out (i.e.
it's useful to compare this with "backslash_quote = off").

I've found it useful to experiment with real-world data with the
optimization dynamically enabled and disabled.

Thoughts?

[1] http://www.postgresql.org/message-id/CAM3SWZSf0Ftxy8QHGAKJh=S80vD2SBx83zkEzuJyZ6R=pTy5xA@mail.gmail.com
--
Peter Geoghegan