pg_visibility The pg_visibility module provides a means for examining the visibility map (VM) and page-level visibility information. These routines return information about three different bits. The all-visible bit in the visibility map indicates that every tuple on a given page of a relation is visible to every current transaction. The all-frozen bit in the visibility map indicates that every tuple on the page is frozen; that is, no future vacuum will need to modify the page until such time as a tuple is inserted, updated, deleted, or locked on that page. The page-level PD_ALL_VISIBLE bit has the same meaning as the all-visible bit in the visibility map, but is stored within the data page itself rather than a separate data structure. These will normally agree, but the page-level bit can sometimes be set while the visibility map bit is clear after a crash recovery; or they can disagree because of a change which occurs after pg_visibility examines the visibility map and before it examines the data page. Functions which display information about PG_ALL_VISIBLE are much more costly than those which only consult the visibility map, because they must read the relation's data blocks rather than only the (much smaller) visibility map. pg_visibility_map(regclass, blkno bigint, all_visible OUT boolean, all_frozen OUT boolean) returns record Returns the all-visible and all-frozen bits in the visibility map for the given block of the given relation. pg_visibility(regclass, blkno bigint, all_visible OUT boolean, all_frozen OUT boolean, pd_all_visible OUT boolean) returns record Returns the all-visible and all-frozen bits in the visibility map for the given block of the given relation, plus the PD_ALL_VISIBILE bit for that block. pg_visibility_map(regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean) returns record Returns the all-visible and all-frozen bits in the visibility map for each block the given relation. pg_visibility(regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean, pd_all_visible OUT boolean) returns record Returns the all-visible and all-frozen bits in the visibility map for each block the given relation, plus the PD_ALL_VISIBLE bit for each block. pg_visibility_map_summary(regclass, all_visible OUT bigint, all_frozen OUT bigint) returns record Returns the number of all-visible pages and the number of all-frozen pages in the relation according to the visibility map. By default, these functions are not publicly executable. Robert Haas rhaas@postgresql.org