diff options
Diffstat (limited to 'src/backend/access/nbtree/nbtxlog.c')
| -rw-r--r-- | src/backend/access/nbtree/nbtxlog.c | 18 |
1 files changed, 16 insertions, 2 deletions
diff --git a/src/backend/access/nbtree/nbtxlog.c b/src/backend/access/nbtree/nbtxlog.c index bba4840da05..0d094ca7faa 100644 --- a/src/backend/access/nbtree/nbtxlog.c +++ b/src/backend/access/nbtree/nbtxlog.c @@ -391,6 +391,19 @@ btree_xlog_vacuum(XLogReaderState *record) BTPageOpaque opaque; /* + * If we are running non-MVCC scans using this index we need to do some + * additional work to ensure correctness, which is known as a "pin scan" + * described in more detail in next paragraphs. We used to do the extra + * work in all cases, whereas we now avoid that work except when the index + * is a toast index, since toast scans aren't fully MVCC compliant. + * If lastBlockVacuumed is set to InvalidBlockNumber then we skip the + * additional work required for the pin scan. + * + * Avoiding this extra work is important since it requires us to touch + * every page in the index, so is an O(N) operation. Worse, it is an + * operation performed in the foreground during redo, so it delays + * replication directly. + * * If queries might be active then we need to ensure every leaf page is * unpinned between the lastBlockVacuumed and the current block, if there * are any. This prevents replay of the VACUUM from reaching the stage of @@ -412,7 +425,7 @@ btree_xlog_vacuum(XLogReaderState *record) * isn't yet consistent; so we need not fear reading still-corrupt blocks * here during crash recovery. */ - if (HotStandbyActiveInReplay()) + if (HotStandbyActiveInReplay() && BlockNumberIsValid(xlrec->lastBlockVacuumed)) { RelFileNode thisrnode; BlockNumber thisblkno; @@ -433,7 +446,8 @@ btree_xlog_vacuum(XLogReaderState *record) * XXX we don't actually need to read the block, we just need to * confirm it is unpinned. If we had a special call into the * buffer manager we could optimise this so that if the block is - * not in shared_buffers we confirm it as unpinned. + * not in shared_buffers we confirm it as unpinned. Optimizing + * this is now moot, since in most cases we avoid the scan. */ buffer = XLogReadBufferExtended(thisrnode, MAIN_FORKNUM, blkno, RBM_NORMAL_NO_LOG); |