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Shared Relation Cache

System catalogs? Pgpool-II needs to access PostgreSQL's system catalogs whenever it recognizes tables in user's query.  For example, Pgpool-II has to know whether the table in question is a temporary table or not. If it's a temporary table, then the query using the temporary table must be routed to the primary PostgreSQL, rather than one of standby PostgreSQL servers because  PostgreSQL does not allow to create temporary tables on standby servers. Another use case is converting the table name to OID (Object Identifier). OIDs are unique keys for objects managed in PostgreSQL's system catalogs.

Same thing can be said to functions. Details of functions, for instance whether they are "immutable" or not, is important information since it affects the decision on which the query result using the function should be cached or not when query cache feature is enabled.
Local query cache for system catalogs Sometimes Pgpool-II needs to issue up to as many as 10 queries …

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