If contrib/btree_gist is used to make a GIST index on a char(N)
(bpchar) column, and that column is retrieved via an index-only
scan, what came out had all trailing spaces removed. Since
that doesn't happen in any other kind of table scan, this is
clearly a bug. The cause is that gbt_bpchar_compress() strips
trailing spaces (using rtrim1) before a new index entry is made.
That was probably a good idea when this code was first written,
but since we invented index-only scans, it's not so good.
One answer could be to mark this opclass as incapable of index-only
scans. But to do so, we'd need an extension module version bump,
followed by manual action by DBAs to install the updated version
of btree_gist. And it's not really a desirable place to end up,
anyway.
Instead, let's fix the code by removing the unwanted space-stripping
action and adjusting the opclass's comparison logic to ignore
trailing spaces as bpchar normally does. This will not hinder
cases that work today, since index searches with this logic will
act the same whether trailing spaces are stored or not. It will
not by itself fix the problem of getting space-stripped results
from index-only scans, of course. Users who care about that can
REINDEX affected indexes after installing this update, to immediately
replace all improperly-truncated index entries. Otherwise, it can
be expected that the index's behavior will change incrementally as
old entries are replaced by new ones.
Per report from Alexander Lakhin. Back-patch to all supported branches.
Discussion: https://postgr.es/m/696c995b-b37f-5526-f45d-04abe713179f@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.
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