diff options
Diffstat (limited to 'src/backend/access/heap/visibilitymap.c')
| -rw-r--r-- | src/backend/access/heap/visibilitymap.c | 11 |
1 files changed, 10 insertions, 1 deletions
diff --git a/src/backend/access/heap/visibilitymap.c b/src/backend/access/heap/visibilitymap.c index 5696abe4d2a..9152c7d1511 100644 --- a/src/backend/access/heap/visibilitymap.c +++ b/src/backend/access/heap/visibilitymap.c @@ -293,6 +293,13 @@ visibilitymap_set(Relation rel, BlockNumber heapBlk, XLogRecPtr recptr, * relation. On return, *buf is a valid buffer with the map page containing * the bit for heapBlk, or InvalidBuffer. The caller is responsible for * releasing *buf after it's done testing and setting bits. + * + * NOTE: This function is typically called without a lock on the heap page, + * so somebody else could change the bit just after we look at it. In fact, + * since we don't lock the visibility map page either, it's even possible that + * someone else could have changed the bit just before we look at it, but yet + * we might see the old value. It is the caller's responsibility to deal with + * all concurrency issues! */ bool visibilitymap_test(Relation rel, BlockNumber heapBlk, Buffer *buf) @@ -327,7 +334,9 @@ visibilitymap_test(Relation rel, BlockNumber heapBlk, Buffer *buf) map = PageGetContents(BufferGetPage(*buf)); /* - * We don't need to lock the page, as we're only looking at a single bit. + * A single-bit read is atomic. There could be memory-ordering effects + * here, but for performance reasons we make it the caller's job to worry + * about that. */ result = (map[mapByte] & (1 << mapBit)) ? true : false; |