Historically, the selectivity functions have simply not distinguished
< from <=, or > from >=, arguing that the fraction of the population that
satisfies the "=" aspect can be considered to be vanishingly small, if the
comparison value isn't any of the most-common-values for the variable.
(If it is, the code path that executes the operator against each MCV will
take care of things properly.) But that isn't really true unless we're
dealing with a continuum of variable values, and in practice we seldom are.
If "x = const" would estimate a nonzero number of rows for a given const
value, then it follows that we ought to estimate different numbers of rows
for "x < const" and "x <= const", even if the const is not one of the MCVs.
Handling this more honestly makes a significant difference in edge cases,
such as the estimate for a tight range (x BETWEEN y AND z where y and z
are close together).
Hence, split scalarltsel into scalarltsel/scalarlesel, and similarly
split scalargtsel into scalargtsel/scalargesel. Adjust <= and >=
operator definitions to reference the new selectivity functions.
Improve the core ineq_histogram_selectivity() function to make a
correction for equality. (Along the way, I learned quite a bit about
exactly why that function gives good answers, which I tried to memorialize
in improved comments.)
The corresponding join selectivity functions were, and remain, just stubs.
But I chose to split them similarly, to avoid confusion and to prevent the
need for doing this exercise again if someone ever makes them less stubby.
In passing, change ineq_histogram_selectivity's clamp for extreme
probability estimates so that it varies depending on the histogram
size, instead of being hardwired at 0.0001. With the default histogram
size of 100 entries, you still get the old clamp value, but bigger
histograms should allow us to put more faith in edge values.
Tom Lane, reviewed by Aleksander Alekseev and Kuntal Ghosh
Discussion: https://postgr.es/m/12232.1499140410@sss.pgh.pa.us
with OPAQUE, as per recent pghackers discussion. I still want to do some
more work on the 'cstring' pseudo-type, but I'm going to commit the bulk
of the changes now before the tree starts shifting under me ...
an 'opclass owner' column in pg_opclass. Nothing is done with it at
present, but since there are plans to invent a CREATE OPERATOR CLASS
command soon, we'll probably want DROP OPERATOR CLASS too, which
suggests that a notion of ownership would be a good idea.
'aggname (aggtype)'. The old syntax 'aggname aggtype' is still accepted
for backwards compatibility. Fix pg_dump, which was actually broken for
most cases of user-defined aggregates. Clean up error messages associated
with these commands.
pgsql-hackers. pg_opclass now has a row for each opclass supported by each
index AM, not a row for each opclass name. This allows pg_opclass to show
directly whether an AM supports an opclass, and furthermore makes it possible
to store additional information about an opclass that might be AM-dependent.
pg_opclass and pg_amop now store "lossy" and "haskeytype" information that we
previously expected the user to remember to provide in CREATE INDEX commands.
Lossiness is no longer an index-level property, but is associated with the
use of a particular operator in a particular index opclass.
Along the way, IndexSupportInitialize now uses the syscaches to retrieve
pg_amop and pg_amproc entries. I find this reduces backend launch time by
about ten percent, at the cost of a couple more special cases in catcache.c's
IndexScanOK.
Initial work by Oleg Bartunov and Teodor Sigaev, further hacking by Tom Lane.
initdb forced.
There's now only one transition value and transition function.
NULL handling in aggregates is a lot cleaner. Also, use Numeric
accumulators instead of integer accumulators for sum/avg on integer
datatypes --- this avoids overflow at the cost of being a little slower.
Implement VARIANCE() and STDDEV() aggregates in the standard backend.
Also, enable new LIKE selectivity estimators by default. Unrelated
change, but as long as I had to force initdb anyway...
(ie, WHERE x > lowbound AND x < highbound). It's not very bright yet
but it does something useful. Also, rename intltsel/intgtsel to
scalarltsel/scalargtsel to reflect usage better. Extend convert_to_scalar
to do something a little bit useful with string data types. Still need
to make it do something with date/time datatypes, but I'll wait for
Thomas's datetime unification dust to settle first. Eventually the
routine ought not have any type-specific knowledge at all; it ought to
be calling a type-dependent routine found via a pg_type column; but
that's a task for another day.
pghackers discussion of 5-Jan-2000. The amopselect and amopnpages
estimators are gone, and in their place is a per-AM amcostestimate
procedure (linked to from pg_am, not pg_amop).
1. Make 'all' works without complaint. Don't have to add the .exp
files to the files list. They are made automagically when
making the respective shared lib file.
Only port that actually uses EXPSUFF (from makefiles/Makefile.*)
is Aix, so if this breaks anybody else, let me know, asap.
2. Make 'clean' actually cleans up correctly. Previously, it would
leave the .o files in C-code directory.
3. Changed references to reflect new location of .c files.
4. Added DELETE statements to complex.source so that it tidies up
when done. Previously, it would leave things in pg_amop,
pg_amproc and pg_opclass. Only possible to do this with the
new SUBSELECT code in 6.3. Nice work, fellas...
Not deleting the index entries would cause a non-fatal error if
complex.sql was run again on the same database. Much tidier now.
5. Corrected the README. obj directory hasn't existed since Bryan
redid the make way back when. Also changed the snipet from psql
to match the current version. POSTGRES95?!? I don't think so. :)
Seem to remember someone posting to one of the lists a while back
that the tutorial code wouldn't compile and/or run. Found four
problems with it that will let it run.
1. Tutorial makefile had a recursive use of DLOBJS.
2. Some tutorial needed semi-colons added to many statements.
3. Complex tutorial didn't clean up after itself.
4. Advanced had a time-travel example. Commented it out and
put a line pointing the user to contrib/spi/README.
code. I have also written a complete complex number package based on this
tutorial; I will submit this as a contribution soon. Is there a particular
format for contributed tar files? I have a C source file, two SQL files, and a
Makefile.
Thomas Lockhart
Bruce Momjian