diff options
| -rw-r--r-- | doc/src/sgml/config.sgml | 9 | ||||
| -rw-r--r-- | src/backend/commands/vacuumlazy.c | 9 |
2 files changed, 18 insertions, 0 deletions
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 4e8c982dd59..c9e0ec2c021 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -2092,6 +2092,15 @@ include_dir 'conf.d' </para> <para> + When this feature is enabled, freed space at the end of a relation + cannot be released to the operating system, since that could remove + information needed to detect the <literal>snapshot too old</> + condition. All space allocated to a relation remains associated with + that relation for reuse only within that relation unless explicitly + freed (for example, with <command>VACUUM FULL</>). + </para> + + <para> This setting does not attempt to guarantee that an error will be generated under any particular circumstances. In fact, if the correct results can be generated from (for example) a cursor which diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c index 4075f4d1c92..231e92d8e46 100644 --- a/src/backend/commands/vacuumlazy.c +++ b/src/backend/commands/vacuumlazy.c @@ -1663,6 +1663,15 @@ lazy_cleanup_index(Relation indrel, * Don't even think about it unless we have a shot at releasing a goodly * number of pages. Otherwise, the time taken isn't worth it. * + * Also don't attempt it if we are doing early pruning/vacuuming, because a + * scan which cannot find a truncated heap page cannot determine that the + * snapshot is too old to read that page. We might be able to get away with + * truncating all except one of the pages, setting its LSN to (at least) the + * maximum of the truncated range if we also treated an index leaf tuple + * pointing to a missing heap page as something to trigger the "snapshot too + * old" error, but that seems fragile and seems like it deserves its own patch + * if we consider it. + * * This is split out so that we can test whether truncation is going to be * called for before we actually do it. If you change the logic here, be * careful to depend only on fields that lazy_scan_heap updates on-the-fly. |