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.