This chapter defines the interface between the core PostgreSQL system and index access methods, which manage individual index types. The core system knows nothing about indexes beyond what is specified here, so it is possible to develop entirely new index types by writing add-on code.
All indexes in PostgreSQL are what are known technically as secondary indexes; that is, the index is physically separate from the table file that it describes. Each index is stored as its own physical relation and so is described by an entry in the pg_class catalog. The contents of an index are entirely under the control of its index access method. In practice, all index access methods divide indexes into standard-size pages so that they can use the regular storage manager and buffer manager to access the index contents. (All the existing index access methods furthermore use the standard page layout described in Section 50.3, and they all use the same format for index tuple headers; but these decisions are not forced on an access method.)
An index is effectively a mapping from some data key values to tuple identifiers, or TIDs, of row versions (tuples) in the index's parent table. A TID consists of a block number and an item number within that block (see Section 50.3). This is sufficient information to fetch a particular row version from the table. Indexes are not directly aware that under MVCC, there may be multiple extant versions of the same logical row; to an index, each tuple is an independent object that needs its own index entry. Thus, an update of a row always creates all-new index entries for the row, even if the key values did not change. Index entries for dead tuples are reclaimed (by vacuuming) when the dead tuples themselves are reclaimed.
Each index access method is described by a row in the pg_am system catalog (see Section 42.3). The principal contents of a pg_am row are references to pg_proc entries that identify the index access functions supplied by the access method. The APIs for these functions are defined later in this chapter. In addition, the pg_am row specifies a few fixed properties of the access method, such as whether it can support multicolumn indexes. There is not currently any special support for creating or deleting pg_am entries; anyone able to write a new access method is expected to be competent to insert an appropriate row for themselves.
To be useful, an index access method must also have one or more operator classes defined in pg_opclass, pg_amop, and pg_amproc. These entries allow the planner to determine what kinds of query qualifications can be used with indexes of this access method. Operator classes are described in Section 32.14, which is prerequisite material for reading this chapter.
An individual index is defined by a pg_class entry that describes it as a physical relation, plus a pg_index entry that shows the logical content of the index — that is, the set of index columns it has and the semantics of those columns, as captured by the associated operator classes. The index columns (key values) can be either simple columns of the underlying table or expressions over the table rows. The index access method normally has no interest in where the index key values come from (it is always handed precomputed key values) but it will be very interested in the operator class information in pg_index. Both of these catalog entries can be accessed as part of the Relation data structure that is passed to all operations on the index.
Some of the flag columns of pg_am have nonobvious implications. The requirements of amcanunique are discussed in Section 48.5, and those of amconcurrent in Section 48.4. The amcanmulticol flag asserts that the access method supports multicolumn indexes, while amoptionalkey asserts that it allows scans where no indexable restriction clause is given for the first index column. When amcanmulticol is false, amoptionalkey essentially says whether the access method allows full-index scans without any restriction clause. Access methods that support multiple index columns must support scans that omit restrictions on any or all of the columns after the first; however they are permitted to require some restriction to appear for the first index column, and this is signaled by setting amoptionalkey false. amindexnulls asserts that index entries are created for NULL key values. Since most indexable operators are strict and hence cannot return TRUE for NULL inputs, it is at first sight attractive to not store index entries for null values: they could never be returned by an index scan anyway. However, this argument fails when an index scan has no restriction clause for a given index column. In practice this means that indexes that have amoptionalkey true must index nulls, since the planner might decide to use such an index with no scan keys at all. A related restriction is that an index access method that supports multiple index columns must support indexing null values in columns after the first, because the planner will assume the index can be used for queries that do not restrict these columns. For example, consider an index on (a,b) and a query with 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. It is, however, OK to omit rows where the first indexed column is null. (GiST currently does so.) Thus, amindexnulls should be set true only if the index access method indexes all rows, including arbitrary combinations of null values.