diff options
| -rw-r--r-- | src/backend/access/nbtree/README | 20 |
1 files changed, 13 insertions, 7 deletions
diff --git a/src/backend/access/nbtree/README b/src/backend/access/nbtree/README index 9692e4cdf64..27f555177ec 100644 --- a/src/backend/access/nbtree/README +++ b/src/backend/access/nbtree/README @@ -429,7 +429,10 @@ allowing subsequent index scans to skip visiting the heap tuple. The "known dead" marking works by setting the index item's lp_flags state to LP_DEAD. This is currently only done in plain indexscans, not bitmap scans, because only plain scans visit the heap and index "in sync" and so -there's not a convenient way to do it for bitmap scans. +there's not a convenient way to do it for bitmap scans. Note also that +LP_DEAD bits are often set when checking a unique index for conflicts on +insert (this is simpler because it takes place when we hold an exclusive +lock on the leaf page). Once an index tuple has been marked LP_DEAD it can actually be removed from the index immediately; since index scans only stop "between" pages, @@ -456,12 +459,15 @@ that this breaks the interlock between VACUUM and indexscans, but that is not so: as long as an indexscanning process has a pin on the page where the index item used to be, VACUUM cannot complete its btbulkdelete scan and so cannot remove the heap tuple. This is another reason why -btbulkdelete has to get a super-exclusive lock on every leaf page, not -only the ones where it actually sees items to delete. So that we can -handle the cases where we attempt LP_DEAD flagging for a page after we -have released its pin, we remember the LSN of the index page when we read -the index tuples from it; we do not attempt to flag index tuples as dead -if the we didn't hold the pin the entire time and the LSN has changed. +btbulkdelete has to get a super-exclusive lock on every leaf page, not only +the ones where it actually sees items to delete. + +LP_DEAD setting by index scans cannot be sure that a TID whose index tuple +it had planned on LP_DEAD-setting has not been recycled by VACUUM if it +drops its pin in the meantime. It must conservatively also remember the +LSN of the page, and only act to set LP_DEAD bits when the LSN has not +changed at all. (Avoiding dropping the pin entirely also makes it safe, of +course.) WAL Considerations ------------------ |