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If the join problem's entire ORDER BY clause can be pushed to the remote server, consider a path that adds this ORDER BY clause. If use_remote_estimate is on, we cost this path using an additional remote EXPLAIN. If not, we just estimate that the path costs 20% more, which is intended to be large enough that we won't request a remote sort when it's not helpful, but small enough that we'll have the remote side do the sort when in doubt. In some cases, the remote sort might actually be free, because the remote query plan might happen to produce output that is ordered the way we need, but without remote estimates we have no way of knowing that. It might also be useful to request sorted output from the remote side if it enables an efficient merge join, but this patch doesn't attempt to handle that case. Ashutosh Bapat with revisions by me. Also reviewed by Fabrízio de Royes Mello and Jeevan Chalke.
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
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User documentation for each module appears in the main SGML
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When building from the source distribution, these modules are not
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Some directories supply new user-defined functions, operators, or
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CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.