F.34. pg_visibility

F.34. pg_visibility
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F.34.1. Functions
F.34.2. Author

+ The pg_visibility module provides a means for examining the + visibility map (VM) and page-level visibility information of a table. + It also provides functions to check the integrity of a visibility map and to + force it to be rebuilt. +

+ Three different bits are used to store information about page-level + visibility. The all-visible bit in the visibility map indicates that every + tuple in the corresponding page of the relation is visible to every current + and future transaction. The all-frozen bit in the visibility map indicates + that every tuple in 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 header's 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 in a separate data structure. + These two bits will normally agree, but the page's all-visible bit can + sometimes be set while the visibility map bit is clear after a crash + recovery. The reported values can also disagree because of a change that + occurs after pg_visibility examines the visibility map and + before it examines the data page. Any event that causes data corruption + can also cause these bits to disagree. +

+ Functions that display information about PD_ALL_VISIBLE bits + are much more costly than those that only consult the visibility map, + because they must read the relation's data blocks rather than only the + (much smaller) visibility map. Functions that check the relation's + data blocks are similarly expensive. +

F.34.1. Functions

pg_visibility_map(relation 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(relation 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_VISIBLE bit of that block. +
pg_visibility_map(relation regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean) returns setof record: + Returns the all-visible and all-frozen bits in the visibility map for + each block of the given relation. +
pg_visibility(relation regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean, pd_all_visible OUT boolean) returns setof record: + Returns the all-visible and all-frozen bits in the visibility map for + each block of the given relation, plus the PD_ALL_VISIBLE + bit of each block. +
pg_visibility_map_summary(relation 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. +
pg_check_frozen(relation regclass, t_ctid OUT tid) returns setof tid: + Returns the TIDs of non-frozen tuples stored in pages marked all-frozen + in the visibility map. If this function returns a non-empty set of + TIDs, the visibility map is corrupt. +
pg_check_visible(relation regclass, t_ctid OUT tid) returns setof tid: + Returns the TIDs of non-all-visible tuples stored in pages marked + all-visible in the visibility map. If this function returns a non-empty + set of TIDs, the visibility map is corrupt. +
pg_truncate_visibility_map(relation regclass) returns void: + Truncates the visibility map for the given relation. This function is + useful if you believe that the visibility map for the relation is + corrupt and wish to force rebuilding it. The first VACUUM + executed on the given relation after this function is executed will scan + every page in the relation and rebuild the visibility map. (Until that + is done, queries will treat the visibility map as containing all zeroes.) +

+ By default, these functions are executable only by superusers and members of the + pg_stat_scan_tables role, with the exception of + pg_truncate_visibility_map(relation regclass) which can only + be executed by superusers. +

F.34.2. Author

+ Robert Haas <rhaas@postgresql.org> +

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