55.5. Row Locking in Foreign Data Wrappers

If an FDW's underlying storage mechanism has a concept of locking individual rows to prevent concurrent updates of those rows, it is usually worthwhile for the FDW to perform row-level locking with as close an approximation as practical to the semantics used in ordinary PostgreSQL tables. There are multiple considerations involved in this.

One key decision to be made is whether to perform early locking or late locking. In early locking, a row is locked when it is first retrieved from the underlying store, while in late locking, the row is locked only when it is known that it needs to be locked. (The difference arises because some rows may be discarded by locally-checked restriction or join conditions.) Early locking is much simpler and avoids extra round trips to a remote store, but it can cause locking of rows that need not have been locked, resulting in reduced concurrency or even unexpected deadlocks. Also, late locking is only possible if the row to be locked can be uniquely re-identified later. Preferably the row identifier should identify a specific version of the row, as PostgreSQL TIDs do.

By default, PostgreSQL ignores locking considerations when interfacing to FDWs, but an FDW can perform early locking without any explicit support from the core code. The API functions described in Section 55.2.5, which were added in PostgreSQL 9.5, allow an FDW to use late locking if it wishes.

An additional consideration is that in READ COMMITTED isolation mode, PostgreSQL may need to re-check restriction and join conditions against an updated version of some target tuple. Rechecking join conditions requires re-obtaining copies of the non-target rows that were previously joined to the target tuple. When working with standard PostgreSQL tables, this is done by including the TIDs of the non-target tables in the column list projected through the join, and then re-fetching non-target rows when required. This approach keeps the join data set compact, but it requires inexpensive re-fetch capability, as well as a TID that can uniquely identify the row version to be re-fetched. By default, therefore, the approach used with foreign tables is to include a copy of the entire row fetched from a foreign table in the column list projected through the join. This puts no special demands on the FDW but can result in reduced performance of merge and hash joins. An FDW that is capable of meeting the re-fetch requirements can choose to do it the first way.

For an UPDATE or DELETE on a foreign table, it is recommended that the ForeignScan operation on the target table perform early locking on the rows that it fetches, perhaps via the equivalent of SELECT FOR UPDATE. An FDW can detect whether a table is an UPDATE/DELETE target at plan time by comparing its relid to root->parse->resultRelation, or at execution time by using ExecRelationIsTargetRelation(). An alternative possibility is to perform late locking within the ExecForeignUpdate or ExecForeignDelete callback, but no special support is provided for this.

For foreign tables that are specified to be locked by a SELECT FOR UPDATE/SHARE command, the ForeignScan operation can again perform early locking by fetching tuples with the equivalent of SELECT FOR UPDATE/SHARE. To perform late locking instead, provide the callback functions defined in Section 55.2.5. In GetForeignRowMarkType, select rowmark option ROW_MARK_EXCLUSIVE, ROW_MARK_NOKEYEXCLUSIVE, ROW_MARK_SHARE, or ROW_MARK_KEYSHARE depending on the requested lock strength. (The core code will act the same regardless of which of these four options you choose.) Elsewhere, you can detect whether a foreign table was specified to be locked by this type of command by using get_plan_rowmark at plan time, or ExecFindRowMark at execution time; you must check not only whether a non-null rowmark struct is returned, but that its strength field is not LCS_NONE.

Lastly, for foreign tables that are used in an UPDATE, DELETE or SELECT FOR UPDATE/SHARE command but are not specified to be row-locked, you can override the default choice to copy entire rows by having GetForeignRowMarkType select option ROW_MARK_REFERENCE when it sees lock strength LCS_NONE. This will cause RefetchForeignRow to be called with that value for markType; it should then re-fetch the row without acquiring any new lock. (If you have a GetForeignRowMarkType function but don't wish to re-fetch unlocked rows, select option ROW_MARK_COPY for LCS_NONE.)

See src/include/nodes/lockoptions.h, the comments for RowMarkType and PlanRowMark in src/include/nodes/plannodes.h, and the comments for ExecRowMark in src/include/nodes/execnodes.h for additional information.