Index Scanning

PostgreSQL 8.2.3 Documentation
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# 49.3. Index Scanning

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 "normalized" form.

The `amgettuple` function has a `direction` argument, which can be either `ForwardScanDirection` (the normal case) or `BackwardScanDirection`. If the first call after `amrescan` specifies `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 call, `amgettuple` must be prepared to advance the scan in either direction from the most recently returned entry.

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 `ammarkpos` call 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 `amgettuple`, an 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 `amgettuple` calls, but we impose several restrictions to simplify matters. In the first place, `amgetmulti` does 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 an `amgetmulti` scan anyway: adjusting the caller's buffered list of TIDs would be complex.) Finally, `amgetmulti` does not guarantee any locking of the returned tuples, with implications spelled out in Section 49.4.

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