There are several settings which can cause the query planner not to generate a parallel query plan under any circumstances. In order for any parallel query plans whatsoever to be generated, the following settings must be configured as indicated.
max_parallel_workers_per_gather must be set to a value
which is greater than zero. This is a special case of the
more general principle that no more workers should be used
than the number configured via
dynamic_shared_memory_type must be set to a value other
none. Parallel query
requires dynamic shared memory in order to pass data
between cooperating processes.
In addition, the system must not be running in single-user mode. Since the entire database system is running in single process in this situation, no background workers will be available.
Even when it is in general possible for parallel query plans to be generated, the planner will not generate them for a given query if any of the following are true:
The query writes any data or locks any database rows. If
a query contains a data-modifying operation either at the
top level or within a CTE, no parallel plans for that query
will be generated. As an exception, the commands
CREATE TABLE ... AS,
SELECT INTO, and
CREATE MATERIALIZED VIEW which create a
new table and populate it can use a parallel plan.
The query might be suspended during execution. In any
situation in which the system thinks that partial or
incremental execution might occur, no parallel plan is
generated. For example, a cursor created using DECLARE
CURSOR will never use a parallel plan. Similarly, a
PL/pgSQL loop of the form
FOR x IN
query LOOP .. END LOOP will never use a parallel
plan, because the parallel query system is unable to verify
that the code in the loop is safe to execute while parallel
query is active.
The query uses any function marked
PARALLEL UNSAFE. Most system-defined
but user-defined functions are marked
PARALLEL UNSAFE by default. See the
discussion of Section 15.4.
The query is running inside of another query that is already parallel. For example, if a function called by a parallel query issues an SQL query itself, that query will never use a parallel plan. This is a limitation of the current implementation, but it may not be desirable to remove this limitation, since it could result in a single query using a very large number of processes.
The transaction isolation level is serializable. This is a limitation of the current implementation.
Even when parallel query plan is generated for a particular
query, there are several circumstances under which it will be
impossible to execute that plan in parallel at execution time. If
this occurs, the leader will execute the portion of the plan
Gather node entirely by
itself, almost as if the
were not present. This will happen if any of the following
conditions are met:
No background workers can be obtained because of the limitation that the total number of background workers cannot exceed max_worker_processes.
No background workers can be obtained because of the limitation that the total number of background workers launched for purposes of parallel query cannot exceed max_parallel_workers.
The client sends an Execute message with a non-zero fetch count. See the discussion of the extended query protocol. Since libpq currently provides no way to send such a message, this can only occur when using a client that does not rely on libpq. If this is a frequent occurrence, it may be a good idea to set max_parallel_workers_per_gather to zero in sessions where it is likely, so as to avoid generating query plans that may be suboptimal when run serially.
The transaction isolation level is serializable. This situation does not normally arise, because parallel query plans are not generated when the transaction isolation level is serializable. However, it can happen if the transaction isolation level is changed to serializable after the plan is generated and before it is executed.
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