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This commit fixes several related problems. All of these problems
escaped testing for the same reason: they do not occur when only
generated advice is supplied, and therefore test_plan_advice was
incapable of detecting these shortcomings.
First, prior to this commit, the advice feedback code never regarded
an unordered sublist of a JOIN_ORDER specification as matching an
unrolled join. As a result, when JOIN_ORDER() advice contained
unordered sublists, the advice feedback tended to be "matched, failed"
even when the advice worked exactly as intended. Only the case where
an unordered sublist was implemented by a plan shape not subject to
unrolling, such as a partitionwise join, worked properly.
Second, prior to this commit, a join order sublist in the initial
position wasn't properly handled. Note that this overlaps with the
problem described in the previous paragraph; JOIN_ORDER({a b} c) was
broken both because of the unordered sublist and because of the
sublist being in the initial position. However, there's more to this
case: JOIN_ORDER((a b) c) means the same as JOIN_ORDER(a b c), but the
advice feedback code didn't know that, and would generate "matched,
failed" for the former case even when everything was working.
Finally, prior to this commit, advice feedback didn't correctly handle
single-element sublists. Those are pretty nonsensical, since by
definition a join involves at least 2 tables, so we could just ban
that case. Instead, at least for now, I've chosen to make advice
feedback handle such cases in the same way that advice enforcement
already does: the extra grouping levels are simply disregarded, so
that JOIN_ORDER({a} ((b))) is enforced in the same way as, and also
gets the same advice feedback as, JOIN_ORDER(a b).
Reported-by: Noah Misch <noah@leadboat.com>
Backpatch-through: 19
Discussion: https://postgr.es/m/CA+TgmoYmXy-jiP5qDhqNEiYFEBzQsArO6O2d9E8szNZqi1bePQ@mail.gmail.com
Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
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.