Per discussion, it's desirable to eliminate all remaining uses of
SnapshotNow, because it has unpleasant semantics: race conditions
can result in seeing multiple versions of a concurrently updated
row, or none at all. By using GetActiveSnapshot() here, callers
will see exactly those rows that would have been visible if the
invoking query had scanned the table using, for example, a SELECT
statement.
This is slightly different from the old behavior, because commits
that happen concurrently with the scan will not affect the results.
In REPEATABLE READ or SERIALIZABLE modes, where transaction
snapshots are used, commits that have happened since the start of
the transaction will also not affect the results. It is hoped
that this minor incompatibility will be thought an improvement,
or at least no worse than what we did before.
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 "gmake all" and "gmake
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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