15.4. Parallel Safety#
The planner classifies operations involved in a query as eitherparallel safe,parallel restricted, orparallel unsafe. A parallel safe operation is one that does not conflict with the use of parallel query. A parallel restricted operation is one that cannot be performed in a parallel worker, but that can be performed in the leader while parallel query is in use. Therefore, parallel restricted operations can never occur below aGather orGather Merge node, but can occur elsewhere in a plan that contains such a node. A parallel unsafe operation is one that cannot be performed while parallel query is in use, not even in the leader. When a query contains anything that is parallel unsafe, parallel query is completely disabled for that query.
The following operations are always parallel restricted:
Scans of common table expressions (CTEs).
Scans of temporary tables.
Scans of foreign tables, unless the foreign data wrapper has an
IsForeignScanParallelSafeAPI that indicates otherwise.Plan nodes that reference a correlated
SubPlan.
15.4.1. Parallel Labeling for Functions and Aggregates#
The planner cannot automatically determine whether a user-defined function or aggregate is parallel safe, parallel restricted, or parallel unsafe, because this would require predicting every operation that the function could possibly perform. In general, this is equivalent to the Halting Problem and therefore impossible. Even for simple functions where it could conceivably be done, we do not try, since this would be expensive and error-prone. Instead, all user-defined functions are assumed to be parallel unsafe unless otherwise marked. When usingCREATE FUNCTION orALTER FUNCTION, markings can be set by specifyingPARALLEL SAFE,PARALLEL RESTRICTED, orPARALLEL UNSAFE as appropriate. When usingCREATE AGGREGATE, thePARALLEL option can be specified withSAFE,RESTRICTED, orUNSAFE as the corresponding value.
Functions and aggregates must be markedPARALLEL UNSAFE if they write to the database, change the transaction state (other than by using a subtransaction for error recovery), access sequences, or make persistent changes to settings. Similarly, functions must be markedPARALLEL RESTRICTED if they access temporary tables, client connection state, cursors, prepared statements, or miscellaneous backend-local state that the system cannot synchronize across workers. For example,setseed andrandom are parallel restricted for this last reason.
In general, if a function is labeled as being safe when it is restricted or unsafe, or if it is labeled as being restricted when it is in fact unsafe, it may throw errors or produce wrong answers when used in a parallel query. C-language functions could in theory exhibit totally undefined behavior if mislabeled, since there is no way for the system to protect itself against arbitrary C code, but in most likely cases the result will be no worse than for any other function. If in doubt, it is probably best to label functions asUNSAFE.
If a function executed within a parallel worker acquires locks that are not held by the leader, for example by querying a table not referenced in the query, those locks will be released at worker exit, not end of transaction. If you write a function that does this, and this behavior difference is important to you, mark such functions asPARALLEL RESTRICTED to ensure that they execute only in the leader.
Note that the query planner does not consider deferring the evaluation of parallel-restricted functions or aggregates involved in the query in order to obtain a superior plan. So, for example, if aWHERE clause applied to a particular table is parallel restricted, the query planner will not consider performing a scan of that table in the parallel portion of a plan. In some cases, it would be possible (and perhaps even efficient) to include the scan of that table in the parallel portion of the query and defer the evaluation of theWHERE clause so that it happens above theGather node. However, the planner does not do this.