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Previously, async.c got rid of duplicate notifications by scanning the list of pending events to compare each one to the proposed new event. This works okay for very small numbers of distinct events, but degrades as O(N^2) for many events. We can improve matters by using a hash table to probe for duplicates. So as not to add a lot of overhead for the simple cases that the code did handle well before, create the hash table only once a (sub)transaction has queued more than 16 distinct notify events. A downside is that we now have to do per-event work to propagate a successful subtransaction's notify events up to its parent. (But this isn't significant unless the subtransaction had many events, in which case the O(N^2) behavior would have been in play already, so we still come out ahead.) We can make some lemonade out of this lemon, though: since we must examine each event anyway, it's now possible to de-duplicate events fully, rather than skipping that for events merged up from subtransactions. Hence, remove the old weasel wording in notify.sgml about whether de-duplication happens or not, and adjust the test case in async-notify.spec that exhibited the old behavior. While at it, rearrange the definition of struct Notification to make it more compact and require just one palloc per event, rather than two or three. This saves space when there are a lot of events, in fact more than enough to buy back the space needed for the hash table. Patch by me, based on discussions around a different patch submitted by Filip Rembiałkowski. Discussion: https://postgr.es/m/17822.1564186806@sss.pgh.pa.us
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.