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Historically, we looked up the target hostname in connectDBStart, so that PQconnectPoll did not need to do DNS name resolution. The patches that added multiple-target-host support to libpq preserved this division of labor; but it's really nonsensical now, because it means that if any one of the target hosts fails to resolve in DNS, the connection fails. That negates the no-single-point-of-failure goal of the feature. Additionally, DNS lookups aren't exactly cheap, but the code did them all even if the first connection attempt succeeds. Hence, rearrange so that PQconnectPoll does the lookups, and only looks up a hostname when it's time to try that host. This does mean that PQconnectPoll could block on a DNS lookup --- but if you wanted to avoid that, you should be using hostaddr, as the documentation has always specified. It seems fairly unlikely that any applications would really care whether the lookup occurs inside PQconnectStart or PQconnectPoll. In addition to calling out that fact explicitly, do some other minor wordsmithing in the docs around the multiple-target-host feature. Since this seems like a bug in the multiple-target-host feature, backpatch to v10 where that was introduced. In the back branches, avoid moving any existing fields of struct pg_conn, just in case any third-party code is looking into that struct. Tom Lane, reviewed by Fabien Coelho Discussion: https://postgr.es/m/4913.1533827102@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/.