When all of the query's DISTINCT pathkeys have been marked as redundant
due to EquivalenceClasses existing which contain constants, we can just
implement the DISTINCT operation on a query by just limiting the number of
returned rows to 1 instead of performing a Unique on all of the matching
(duplicate) rows.
This applies in cases such as:
SELECT DISTINCT col,col2 FROM tab WHERE col = 1 AND col2 = 10;
If there are any matching rows, then they must all be {1,10}. There's no
point in fetching all of those and running a Unique operator on them to
leave only a single row. Here we effectively just find the first row and
then stop. We are obviously unable to apply this optimization if either
the col = 1 or col2 = 10 were missing from the WHERE clause or if there
were any additional columns in the SELECT clause.
Such queries are probably not all that common, but detecting when we can
apply this optimization amounts to checking if the distinct_pathkeys are
NULL, which is very cheap indeed.
Nothing is done here to check if the query already has a LIMIT clause. If
it does then the plan may end up with 2 Limits nodes. There's no harm in
that and it's probably not worth the complexity to unify them into a
single Limit node.
Author: David Rowley
Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/CAApHDvqS0j8RUWRUSgCAXxOqnYjHUXmKwspRj4GzVfOO25ByHA@mail.gmail.com
Discussion: https://postgr.es/m/MEYPR01MB7101CD5DA0A07C9DE2B74850A4239@MEYPR01MB7101.ausprd01.prod.outlook.com
The idea behind this patch is to make it possible to run individual
test scripts without running the entire core test suite. Making all
the scripts completely independent would involve a massive rewrite,
and would probably be worse for coverage of things like concurrent DDL.
So this patch just does what seems practical with limited changes.
The net effect is that any test script can be run after running
limited earlier dependencies:
* all scripts depend on test_setup
* many scripts depend on create_index
* other dependencies are few in number, and are documented in
the parallel_schedule file.
To accomplish this, I chose a small number of commonly-used tables
and moved their creation and filling into test_setup. Later scripts
are expected not to modify these tables' data contents, for fear of
affecting other scripts' results. Also, our former habit of declaring
all C functions in one place is now gone in favor of declaring them
where they're used, if that's just one script, or in test_setup if
necessary.
There's more that could be done to remove some of the remaining
inter-script dependencies, but significantly more-invasive changes
would be needed, and at least for now it doesn't seem worth it.
Discussion: https://postgr.es/m/1114748.1640383217@sss.pgh.pa.us
In this case we generate two PathKey references to the expression (one for
DISTINCT and one for ORDER BY) and they really need to refer to the same
EquivalenceClass. However get_eclass_for_sort_expr was being overly paranoid
and creating two different EC's. Correct behavior is to use the SortGroupRef
index to decide whether two references to volatile expressions that are
equal() (ie textually equivalent) should be considered the same.
Backpatch to 8.4. Possibly this should be changed in 8.3 as well, but
I'll refrain in the absence of evidence of a visible failure in that branch.
Per bug #5049.
SELECT DISTINCT ON (expr [, expr ...]) targetlist ...
and there is a check to make sure that the user didn't specify an ORDER BY
that's incompatible with the DISTINCT operation.
Reimplement nodeUnique and nodeGroup to use the proper datatype-specific
equality function for each column being compared --- they used to do
bitwise comparisons or convert the data to text strings and strcmp().
(To add insult to injury, they'd look up the conversion functions once
for each tuple...) Parse/plan representation of DISTINCT is now a list
of SortClause nodes.
initdb forced by querytree change...
David Rowley
Tom Lane
Thomas G. Lockhart
Bruce Momjian
Marc G. Fournier