Each index access method is described by a row in the pg_am system catalog. The pg_am entry specifies a name and a handler function for the index access method. These entries can be created and deleted using the CREATE ACCESS METHOD and DROP ACCESS METHOD SQL commands.
An index access method handler function must be declared to accept a single argument of type internal and to return the pseudo-type index_am_handler. The argument is a dummy value that simply serves to prevent handler functions from being called directly from SQL commands. The result of the function must be a palloc'd struct of type IndexAmRoutine, which contains everything that the core code needs to know to make use of the index access method. The IndexAmRoutine struct, also called the access method's API struct, includes fields specifying assorted fixed properties of the access method, such as whether it can support multicolumn indexes. More importantly, it contains pointers to support functions for the access method, which do all of the real work to access indexes. These support functions are plain C functions and are not visible or callable at the SQL level. The support functions are described in Section 62.2.
The structure IndexAmRoutine is defined thus:
typedef struct IndexAmRoutine { NodeTag type; /* * Total number of strategies (operators) by which we can traverse/search * this AM. Zero if AM does not have a fixed set of strategy assignments. */ uint16 amstrategies; /* total number of support functions that this AM uses */ uint16 amsupport; /* opclass options support function number or 0 */ uint16 amoptsprocnum; /* does AM support ORDER BY indexed column's value? */ bool amcanorder; /* does AM support ORDER BY result of an operator on indexed column? */ bool amcanorderbyop; /* does AM support backward scanning? */ bool amcanbackward; /* does AM support UNIQUE indexes? */ bool amcanunique; /* does AM support multi-column indexes? */ bool amcanmulticol; /* does AM require scans to have a constraint on the first index column? */ bool amoptionalkey; /* does AM handle ScalarArrayOpExpr quals? */ bool amsearcharray; /* does AM handle IS NULL/IS NOT NULL quals? */ bool amsearchnulls; /* can index storage data type differ from column data type? */ bool amstorage; /* can an index of this type be clustered on? */ bool amclusterable; /* does AM handle predicate locks? */ bool ampredlocks; /* does AM support parallel scan? */ bool amcanparallel; /* does AM support parallel build? */ bool amcanbuildparallel; /* does AM support columns included with clause INCLUDE? */ bool amcaninclude; /* does AM use maintenance_work_mem? */ bool amusemaintenanceworkmem; /* does AM summarize tuples, with at least all tuples in the block * summarized in one summary */ bool amsummarizing; /* OR of parallel vacuum flags */ uint8 amparallelvacuumoptions; /* type of data stored in index, or InvalidOid if variable */ Oid amkeytype; /* interface functions */ ambuild_function ambuild; ambuildempty_function ambuildempty; aminsert_function aminsert; aminsertcleanup_function aminsertcleanup; ambulkdelete_function ambulkdelete; amvacuumcleanup_function amvacuumcleanup; amcanreturn_function amcanreturn; /* can be NULL */ amcostestimate_function amcostestimate; amoptions_function amoptions; amproperty_function amproperty; /* can be NULL */ ambuildphasename_function ambuildphasename; /* can be NULL */ amvalidate_function amvalidate; amadjustmembers_function amadjustmembers; /* can be NULL */ ambeginscan_function ambeginscan; amrescan_function amrescan; amgettuple_function amgettuple; /* can be NULL */ amgetbitmap_function amgetbitmap; /* can be NULL */ amendscan_function amendscan; ammarkpos_function ammarkpos; /* can be NULL */ amrestrpos_function amrestrpos; /* can be NULL */ /* interface functions to support parallel index scans */ amestimateparallelscan_function amestimateparallelscan; /* can be NULL */ aminitparallelscan_function aminitparallelscan; /* can be NULL */ amparallelrescan_function amparallelrescan; /* can be NULL */ } IndexAmRoutine;
To be useful, an index access method must also have one or more operator families and operator classes defined in pg_opfamily, 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 families and classes are described in Section 36.16, 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 fields of IndexAmRoutine have nonobvious
implications. The requirements of amcanunique
are discussed in Section 62.5.
The amcanmulticol
flag asserts that the
access method supports multi-key-column 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 supports 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.
One reason that an index AM might set
amoptionalkey
false is if it doesn't index
null 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.
An index access method that does index nulls may also set
amsearchnulls
, indicating that it supports
IS NULL
and IS NOT NULL
clauses as search
conditions.
The amcaninclude
flag indicates whether the
access method supports “included” columns, that is it can
store (without processing) additional columns beyond the key column(s).
The requirements of the preceding paragraph apply only to the key
columns. In particular, the combination
of amcanmulticol
=false
and amcaninclude
=true
is
sensible: it means that there can only be one key column, but there can
also be included column(s). Also, included columns must be allowed to be
null, independently of amoptionalkey
.
The amsummarizing
flag indicates whether the
access method summarizes the indexed tuples, with summarizing granularity
of at least per block.
Access methods that do not point to individual tuples, but to block ranges
(like BRIN), may allow the HOT optimization
to continue. This does not apply to attributes referenced in index
predicates, an update of such an attribute always disables HOT.