Greenplum System Catalog Maintenance

Numerous database updates with CREATE and DROP commands increase the system catalog size and affect system performance. For example, running many DROP TABLE statements degrades the overall system performance due to excessive data scanning during metadata operations on catalog tables. The performance loss occurs between thousands to tens of thousands of DROP TABLE statements depending on the system.

Greenplum recommends you regularly run a system catalog maintenance procedure to reclaim the space occupied by deleted objects. If a regular procedure has not been run for a long time, you may need to run a more intensive procedure to clear the system catalog. This section describes both procedures.

Regular System Catalog Maintenance
It is recommended that you periodically run VACUUM on the system catalog to clear the space that deleted objects occupy. If regular database operations include numerous DROP statements, it is safe and appropriate to run a system catalog maintenance procedure with VACUUM daily at off-peak hours. You can do this while the system is available.

The following example script performs a VACUUM of the Greenplum Database system catalog:

psql -tc "select '$VCOMMAND' || ' pg_catalog.' || relname || ';' from pg_class a,pg_namespace b where a.relnamespace=b.oid and b.nspname= 'pg_catalog' and a.relkind='r'" $DBNAME | psql -a $DBNAME

Intensive System Catalog Maintenance
If a system catalog maintenance procedure has not been performed in a long time, the catalog can become bloated with dead space; this causes excessively long wait times for simple metadata operations. A wait of more than two seconds to list user tables, such as with the \d metacommand from within psql, is an indication of catalog bloat.

If you see indications of system catalog bloat, you must perform an intensive system catalog maintenance procedure with VACUUM FULL during a scheduled downtime period. During this period, stop all catalog activity on the system; the FULL system catalog maintenance procedure takes exclusive locks against the system catalog.

Running regular system catalog maintenance procedures can prevent the need for this more costly procedure.