diff options
Diffstat (limited to 'src/backend/executor/tqueue.c')
| -rw-r--r-- | src/backend/executor/tqueue.c | 22 |
1 files changed, 14 insertions, 8 deletions
diff --git a/src/backend/executor/tqueue.c b/src/backend/executor/tqueue.c index e81c333e4cd..64555599cee 100644 --- a/src/backend/executor/tqueue.c +++ b/src/backend/executor/tqueue.c @@ -524,13 +524,18 @@ DestroyTupleQueueReader(TupleQueueReader *reader) /* * Fetch a tuple from a tuple queue reader. * + * The return value is NULL if there are no remaining tuples or if + * nowait = true and no tuple is ready to return. *done, if not NULL, + * is set to true when there are no remaining tuples and otherwise to false. + * + * The returned tuple, if any, is allocated in CurrentMemoryContext. + * That should be a short-lived (tuple-lifespan) context, because we are + * pretty cavalier about leaking memory in that context if we have to do + * tuple remapping. + * * Even when shm_mq_receive() returns SHM_MQ_WOULD_BLOCK, this can still * accumulate bytes from a partially-read message, so it's useful to call * this with nowait = true even if nothing is returned. - * - * The return value is NULL if there are no remaining queues or if - * nowait = true and no tuple is ready to return. *done, if not NULL, - * is set to true when queue is detached and otherwise to false. */ HeapTuple TupleQueueReaderNext(TupleQueueReader *reader, bool nowait, bool *done) @@ -565,10 +570,11 @@ TupleQueueReaderNext(TupleQueueReader *reader, bool nowait, bool *done) * OK, we got a message. Process it. * * One-byte messages are mode switch messages, so that we can switch - * between "control" and "data" mode. When in "data" mode, each - * message (unless exactly one byte) is a tuple. When in "control" - * mode, each message provides a transient-typmod-to-tupledesc mapping - * so we can interpret future tuples. + * between "control" and "data" mode. Otherwise, when in "data" mode, + * each message is a tuple. When in "control" mode, each message + * provides a transient-typmod-to-tupledesc mapping to let us + * interpret future tuples. Both of those cases certainly require + * more than one byte, so no confusion is possible. */ if (nbytes == 1) { |