|PostgreSQL 8.2.23 Documentation|
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In an index scan, the index access method is responsible for regurgitating the TIDs of all the tuples it has been told about that match the scan keys. The access method is not involved in actually fetching those tuples from the index's parent table, nor in determining whether they pass the scan's time qualification test or other conditions.
A scan key is the internal representation of a WHERE clause of the form index_key operator constant, where the index key is one of the columns of the index and the operator is one of the members of the operator class associated with that index column. An index scan has zero or more scan keys, which are implicitly ANDed — the returned tuples are expected to satisfy all the indicated conditions.
The operator class may indicate that the index is lossy for a particular operator; this implies that the index scan will return all the entries that pass the scan key, plus possibly additional entries that do not. The core system's index-scan machinery will then apply that operator again to the heap tuple to verify whether or not it really should be selected. For non-lossy operators, the index scan must return exactly the set of matching entries, as there is no recheck.
Note that it is entirely up to the access method to ensure
that it correctly finds all and only the entries passing all the
given scan keys. Also, the core system will simply hand off all
the WHERE clauses that match the index
keys and operator classes, without any semantic analysis to
determine whether they are redundant or contradictory. As an
example, given WHERE x > 4 AND x >
14 where x is a b-tree indexed
column, it is left to the b-tree
amrescan function to realize that the first
scan key is redundant and can be discarded. The extent of
preprocessing needed during
amrescan will depend on the extent to which the
index access method needs to reduce the scan keys to a
amgettuple function has a
direction argument, which can be either
ForwardScanDirection (the normal case)
or BackwardScanDirection. If the first
BackwardScanDirection, then the set of
matching index entries is to be scanned back-to-front rather than
in the normal front-to-back direction, so
amgettuple must return the last matching tuple
in the index, rather than the first one as it normally would.
(This will only occur for access methods that advertise they
support ordered scans by setting pg_am.amorderstrategy nonzero.) After the first
amgettuple must be prepared
to advance the scan in either direction from the most recently
The access method must support "marking" a position in a scan and later returning
to the marked position. The same position may be restored
multiple times. However, only one position need be remembered per
scan; a new
overrides the previously marked position.
Both the scan position and the mark position (if any) must be maintained consistently in the face of concurrent insertions or deletions in the index. It is OK if a freshly-inserted entry is not returned by a scan that would have found the entry if it had existed when the scan started, or for the scan to return such an entry upon rescanning or backing up even though it had not been returned the first time through. Similarly, a concurrent delete may or may not be reflected in the results of a scan. What is important is that insertions or deletions not cause the scan to miss or multiply return entries that were not themselves being inserted or deleted.
Instead of using
index scan can be done with
amgetmulti to fetch multiple tuples per call.
This can be noticeably more efficient than
amgettuple because it allows avoiding
lock/unlock cycles within the access method. In principle
amgetmulti should have the same
effects as repeated
calls, but we impose several restrictions to simplify matters. In
the first place,
not take a direction argument, and
therefore it does not support backwards scan nor intrascan
reversal of direction. The access method need not support marking
or restoring scan positions during an
amgetmulti scan, either. (These restrictions
cost little since it would be difficult to use these features in
amgetmulti scan anyway:
adjusting the caller's buffered list of TIDs would be complex.)
amgetmulti does not
guarantee any locking of the returned tuples, with implications
spelled out in Section 49.4.