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Previously, when a path type was disabled by e.g. enable_seqscan=false, we either avoided generating that path type in the first place, or more commonly, we added a large constant, called disable_cost, to the estimated startup cost of that path. This latter approach can distort planning. For instance, an extremely expensive non-disabled path could seem to be worse than a disabled path, especially if the full cost of that path node need not be paid (e.g. due to a Limit). Or, as in the regression test whose expected output changes with this commit, the addition of disable_cost can make two paths that would normally be distinguishible in cost seem to have fuzzily the same cost. To fix that, we now count the number of disabled path nodes and consider that a high-order component of both the startup cost and the total cost. Hence, the path list is now sorted by disabled_nodes and then by total_cost, instead of just by the latter, and likewise for the partial path list. It is important that this number is a count and not simply a Boolean; else, as soon as we're unable to respect disabled path types in all portions of the path, we stop trying to avoid them where we can. Because the path list is now sorted by the number of disabled nodes, the join prechecks must compute the count of disabled nodes during the initial cost phase instead of postponing it to final cost time. Counts of disabled nodes do not cross subquery levels; at present, there is no reason for them to do so, since the we do not postpone path selection across subquery boundaries (see make_subplan). Reviewed by Andres Freund, Heikki Linnakangas, and David Rowley. Discussion: http://postgr.es/m/CA+TgmoZ_+MS+o6NeGK2xyBv-xM+w1AfFVuHE4f_aq6ekHv7YSQ@mail.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.