07e4d03fb4
If some existing listener is far behind, incoming new listener sessions would start from that session's read pointer and then need to advance over many already-committed notification messages, which they have no interest in. This was expensive in itself and also thrashed the pg_notify SLRU buffers a lot more than necessary. We can improve matters considerably in typical scenarios, without much added cost, by starting from the furthest-ahead read pointer, not the furthest-behind one. We do have to consider only sessions in our own database when doing this, which requires an extra field in the data structure, but that's a pretty small cost. Back-patch to 9.0 where the current LISTEN/NOTIFY logic was introduced. Matt Newell, slightly adjusted by me
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: http://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 http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.