diff options
| -rw-r--r-- | doc/src/sgml/maintenance.sgml | 32 | ||||
| -rw-r--r-- | doc/src/sgml/storage.sgml | 15 |
2 files changed, 42 insertions, 5 deletions
diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml index 03cc6c987a2..93c3ff5f2bb 100644 --- a/doc/src/sgml/maintenance.sgml +++ b/doc/src/sgml/maintenance.sgml @@ -102,6 +102,11 @@ </listitem> <listitem> + <simpara>To update the visibility map, which speeds up index-only + scans.</simpara> + </listitem> + + <listitem> <simpara>To protect against loss of very old data due to <firstterm>transaction ID wraparound</>.</simpara> </listitem> @@ -329,6 +334,33 @@ </tip> </sect2> + <sect2 id="vacuum-for-visibility-map"> + <title>Updating The Visibility Map</title> + + <para> + Vacuum maintains a <link linkend="storage-vm">visibility map</> for each + table to keep track of which pages contain only tuples that are known to be + visible to all active transactions (and all future transactions, until the + page is again modified). This has two purposes. First, vacuum + itself can skip such pages on the next run, since there is nothing to + clean up. + </para> + + <para> + Second, it allows <productname>PostgreSQL</productname> to answer some + queries using only the index, without reference to the underlying table. + Since <productname>PostgreSQL</productname> indexes don't contain tuple + visibility information, a normal index scan fetches the heap tuple for each + matching index entry, to check whether it should be seen by the current + transaction. An <firstterm>index-only scan</>, on the other hand, checks + the visibility map first. If it's known that all tuples on the page are + visible, the heap fetch can be skipped. This is most noticeable on + large data sets where the visibility map can prevent disk accesses. + The visibility map is vastly smaller than the heap, so it can easily be + cached even when the heap is very large. + </para> + </sect2> + <sect2 id="vacuum-for-wraparound"> <title>Preventing Transaction ID Wraparound Failures</title> diff --git a/doc/src/sgml/storage.sgml b/doc/src/sgml/storage.sgml index 45223f563d7..bd2dca39fcb 100644 --- a/doc/src/sgml/storage.sgml +++ b/doc/src/sgml/storage.sgml @@ -494,11 +494,16 @@ Note that indexes do not have VMs. <para> The visibility map simply stores one bit per heap page. A set bit means that all tuples on the page are known to be visible to all transactions. -This means that the page does not contain any tuples that need to be vacuumed; -in future it might also be used to avoid visiting the page for visibility -checks. The map is conservative in the sense that we -make sure that whenever a bit is set, we know the condition is true, but if -a bit is not set, it might or might not be true. +This means that the page does not contain any tuples that need to be vacuumed. +This information can also be used by <firstterm>index-only scans</> to answer +queries using only the index tuple. +</para> + +<para> +The map is conservative in the sense that we make sure that whenever a bit is +set, we know the condition is true, but if a bit is not set, it might or +might not be true. Visibility map bits are only set by vacuum, but are +cleared by any data-modifying operations on a page. </para> </sect1> |