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Until now PostgreSQL has not been very smart about optimizing away IS NOT NULL base quals on columns defined as NOT NULL. The evaluation of these needless quals adds overhead. Ordinarily, anyone who came complaining about that would likely just have been told to not include the qual in their query if it's not required. However, a recent bug report indicates this might not always be possible. Bug 17540 highlighted that when we optimize Min/Max aggregates the IS NOT NULL qual that the planner adds to make the rewritten plan ignore NULLs can cause issues with poor index choice. That particular case demonstrated that other quals, especially ones where no statistics are available to allow the planner a chance at estimating an approximate selectivity for can result in poor index choice due to cheap startup paths being prefered with LIMIT 1. Here we take generic approach to fixing this by having the planner check for NOT NULL columns and just have the planner remove these quals (when they're not needed) for all queries, not just when optimizing Min/Max aggregates. Additionally, here we also detect IS NULL quals on a NOT NULL column and transform that into a gating qual so that we don't have to perform the scan at all. This also works for join relations when the Var is not nullable by any outer join. This also helps with the self-join removal work as it must replace strict join quals with IS NOT NULL quals to ensure equivalence with the original query. Author: David Rowley, Richard Guo, Andy Fan Reviewed-by: Richard Guo, David Rowley Discussion: https://postgr.es/m/CAApHDvqg6XZDhYRPz0zgOcevSMo0d3vxA9DvHrZtKfqO30WTnw@mail.gmail.com Discussion: https://postgr.es/m/17540-7aa1855ad5ec18b4%40postgresql.org
The PostgreSQL contrib tree
---------------------------
This subtree contains porting tools, analysis utilities, and plug-in
features that are not part of the core PostgreSQL system, mainly
because they address a limited audience or are too experimental to be
part of the main source tree. This does not preclude their
usefulness.
User documentation for each module appears in the main SGML
documentation.
When building from the source distribution, these modules are not
built automatically, unless you build the "world" target. You can
also build and install them all by running "make all" and "make
install" in this directory; or to build and install just one selected
module, do the same in that module's subdirectory.
Some directories supply new user-defined functions, operators, or
types. To make use of one of these modules, after you have installed
the code you need to register the new SQL objects in the database
system by executing a CREATE EXTENSION command. In a fresh database,
you can simply do
CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.