This section provides an overview of the page format used by Postgres tables. User-defined access methods need not use this page format.
In the following explanation, a byte is assumed to contain 8 bits. In addition, the term item refers to data that is stored in Postgres tables.
The following table shows how pages in both normal Postgres tables and Postgres indices (e.g., a B-tree index) are structured.
Table 7-1. Sample Page Layout
The first 8 bytes of each page consists of a page header (PageHeaderData). Within the header, the first three 2-byte integer fields (lower, upper, and special) represent byte offsets to the start of unallocated space, to the end of unallocated space, and to the start of special space. Special space is a region at the end of the page that is allocated at page initialization time and contains information specific to an access method. The last 2 bytes of the page header, opaque, encode the page size and information on the internal fragmentation of the page. Page size is stored in each page because frames in the buffer pool may be subdivided into equal sized pages on a frame by frame basis within a table. The internal fragmentation information is used to aid in determining when page reorganization should occur.
Following the page header are item identifiers (ItemIdData). New item identifiers are allocated from the first four bytes of unallocated space. Because an item identifier is never moved until it is freed, its index may be used to indicate the location of an item on a page. In fact, every pointer to an item (ItemPointer) created by Postgres consists of a frame number and an index of an item identifier. An item identifier contains a byte-offset to the start of an item, its length in bytes, and a set of attribute bits which affect its interpretation.
The items themselves are stored in space allocated backwards from the end of unallocated space. Usually, the items are not interpreted. However when the item is too long to be placed on a single page or when fragmentation of the item is desired, the item is divided and each piece is handled as distinct items in the following manner. The first through the next to last piece are placed in an item continuation structure (ItemContinuationData). This structure contains itemPointerData which points to the next piece and the piece itself. The last piece is handled normally.
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