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Stats kinds can set a new option called "track_entry_count" (disabled by default, available for variable-numbered stats) that will make pgstats track the number of entries that exist in its shared hashtable. As there is only one code path where a new entry is added, and one code path where entries are freed, the count tracking is straight-forward in its implementation. Reads of these counters are optimistic, and may change across two calls. The counter is incremented when an entry is created (not when reused), and is decremented when an entry is freed from the hashtable (marked for drop with its refcount reaching 0), which is something that pgstats decides internally. A first use case of this facility would be pg_stat_statements, where we need to be able to cap the number of entries that would be stored in the shared hashtable, based on its "max" GUC. The module currently relies on hash_get_num_entries(), which offers a cheap way to count how many entries are in its hash table, but we cannot do that in pgstats for variable-sized stats kinds as a single hashtable is used for all the stats kinds. Independently of PGSS, this is useful for other custom stats kinds that want to cap, control, or track the number of entries they have, without depending on a potentially expensive sequential scan to know the number of entries while holding an extra exclusive lock. Author: Michael Paquier <michael@paquier.xyz> Reviewed-by: Sami Imseih <samimseih@gmail.com> Reviewed-by: Keisuke Kuroda <keisuke.kuroda.3862@gmail.com> Reviewed-by: Chao Li <li.evan.chao@gmail.com> Discussion: https://postgr.es/m/aMPKWR81KT5UXvEr@paquier.xyz
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.
Copyright and license information can be found in the file COPYRIGHT.
General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.
The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.