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Since 9.5, it's possible that some but not all columns of an index support returning the indexed value for index-only scans. If the same indexed column appears in index columns that behave both ways, check_index_only() supposed that it'd be OK to do an index-only scan testing that column; but that fails if we have to recheck the indexed condition on one of the columns that doesn't support this. In principle we could make this work by remapping the recheck expressions to pull the value from a column that does support returning the indexed value. But such cases are so weird and rare that, at least for now, it doesn't seem worth the trouble. Instead, just teach check_index_only that a value is returnable only if all the index columns containing it are returnable, rather than any of them. Per report from David Pereiro Lagares. Back-patch to 9.5 where the possibility of this situation appeared. Kyotaro Horiguchi Discussion: https://postgr.es/m/1516210494.1798.16.camel@nlpgo.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.