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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
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| .. | ||
| authentication | ||
| examples | ||
| icu | ||
| isolation | ||
| kerberos | ||
| ldap | ||
| locale | ||
| mb | ||
| modules | ||
| perl | ||
| recovery | ||
| regress | ||
| ssl | ||
| subscription | ||
| Makefile | ||
| meson.build | ||
| README | ||
PostgreSQL tests ================ This directory contains a variety of test infrastructure as well as some of the tests in PostgreSQL. Not all tests are here -- in particular, there are more in individual contrib/ modules and in src/bin. Not all these tests get run by "make check". Check src/test/Makefile to see which tests get run automatically. authentication/ Tests for authentication (but see also below) examples/ Demonstration programs for libpq that double as regression tests via "make check" isolation/ Tests for concurrent behavior at the SQL level kerberos/ Tests for Kerberos/GSSAPI authentication and encryption ldap/ Tests for LDAP-based authentication locale/ Sanity checks for locale data, encodings, etc mb/ Tests for multibyte encoding (UTF-8) support modules/ Extensions used only or mainly for test purposes, generally not suitable for installing in production databases perl/ Infrastructure for Perl-based TAP tests recovery/ Test suite for recovery and replication regress/ PostgreSQL's main regression test suite, pg_regress ssl/ Tests to exercise and verify SSL certificate handling subscription/ Tests for logical replication