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Diffstat (limited to 'doc/src/sgml/storage.sgml')
| -rw-r--r-- | doc/src/sgml/storage.sgml | 39 |
1 files changed, 36 insertions, 3 deletions
diff --git a/doc/src/sgml/storage.sgml b/doc/src/sgml/storage.sgml index 8e4df1bdfff..ee7f814ea89 100644 --- a/doc/src/sgml/storage.sgml +++ b/doc/src/sgml/storage.sgml @@ -1,4 +1,4 @@ -<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.28 2009/05/16 22:03:53 tgl Exp $ --> +<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.29 2009/06/17 21:58:49 tgl Exp $ --> <chapter id="storage"> @@ -135,8 +135,9 @@ main file (a/k/a main fork), each table and index has a <firstterm>free space map</> (see <xref linkend="storage-fsm">), which stores information about free space available in the relation. The free space map is stored in a file named with the filenode number plus the suffix <literal>_fsm</>. Tables also have a -visibility map fork, with the suffix <literal>_vm</>, to track which pages are -known to have no dead tuples and therefore need no vacuuming. +<firstterm>visibility map</>, stored in a fork with the suffix +<literal>_vm</>, to track which pages are known to have no dead tuples. +The visibility map is described further in <xref linkend="storage-vm">. </para> <caution> @@ -417,6 +418,38 @@ information stored in free space maps (see <xref linkend="pgfreespacemap">). </sect1> +<sect1 id="storage-vm"> + +<title>Visibility Map</title> + +<indexterm> + <primary>Visibility Map</primary> +</indexterm> +<indexterm><primary>VM</><see>Visibility Map</></indexterm> + +<para> +Each heap relation has a Visibility Map +(VM) to keep track of which pages contain only tuples that are known to be +visible to all active transactions. It's stored +alongside the main relation data in a separate relation fork, named after the +filenode number of the relation, plus a <literal>_vm</> suffix. For example, +if the filenode of a relation is 12345, the VM is stored in a file called +<filename>12345_vm</>, in the same directory as the main relation file. +Note that indexes do not have VMs. +</para> + +<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. +</para> + +</sect1> + <sect1 id="storage-page-layout"> <title>Database Page Layout</title> |