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3.3. pg_am

pg_am stores information about index access methods. There is one row for each index access method supported by the system.

Table 3-3. pg_am Columns

Name Type References Description
amname name   name of the access method
amowner int4 pg_shadow.usesysid user ID of the owner (currently not used)
amstrategies int2   number of operator strategies for this access method
amsupport int2   number of support routines for this access method
amorderstrategy int2   zero if the index offers no sort order, otherwise the strategy number of the strategy operator that describes the sort order
amcanunique bool   does AM support unique indexes?
amcanmulticol bool   does AM support multicolumn indexes?
amindexnulls bool   does AM support NULL index entries?
amconcurrent bool   does AM support concurrent updates?
amgettuple regproc pg_proc.oid "next valid tuple" function
aminsert regproc pg_proc.oid "insert this tuple" function
ambeginscan regproc pg_proc.oid "start new scan" function
amrescan regproc pg_proc.oid "restart this scan" function
amendscan regproc pg_proc.oid "end this scan" function
ammarkpos regproc pg_proc.oid "mark current scan position" function
amrestrpos regproc pg_proc.oid "restore marked scan position" function
ambuild regproc pg_proc.oid "build new index" function
ambulkdelete regproc pg_proc.oid bulk-delete function
amcostestimate regproc pg_proc.oid estimate cost of an indexscan

An index AM that supports multiple columns (has amcanmulticol true) must support indexing nulls in columns after the first, because the planner will assume the index can be used for queries on just the first column(s). For example, consider an index on (a,b) and a query WHERE a = 4. The system will assume the index can be used to scan for rows with a = 4, which is wrong if the index omits rows where b is null. However it is okay to omit rows where the first indexed column is null. (GiST currently does so.) amindexnulls should be set true only if the index AM indexes all rows, including arbitrary combinations of nulls.