This section provides an overview of the page format used by PostgreSQL tables and indexes. (Index access methods need not use this page format. At present, all index methods do use this basic format, but the data kept on index metapages usually doesn't follow the item layout rules exactly.) TOAST tables and sequences are formatted just like a regular table.
In the following explanation, a byte is assumed to contain 8 bits. In addition, the term item refers to an individual data value that is stored on a page. In a table, an item is a tuple (row); in an index, an item is an index entry.
Table 7-1 shows the basic layout of a page. There are five parts to each page.
Table 7-1. Sample Page Layout
|PageHeaderData||20 bytes long. Contains general information about the page, including free space pointers.|
|ItemPointerData||Array of (offset,length) pairs pointing to the actual items.|
|Free space||The unallocated space. All new tuples are allocated from here, generally from the end.|
|Items||The actual items themselves.|
|Special Space||Index access method specific data. Different methods store different data. Empty in ordinary tables.|
The first 20 bytes of each page consists of a page header (PageHeaderData). Its format is detailed in Table 7-2. The first two fields deal with WAL related stuff. This is followed by three 2-byte integer fields (pd_lower, pd_upper, and pd_special). These represent byte offsets to the start of unallocated space, to the end of unallocated space, and to the start of the special space.
Table 7-2. PageHeaderData Layout
|pd_lsn||XLogRecPtr||8 bytes||LSN: next byte after last byte of xlog|
|pd_sui||StartUpID||4 bytes||SUI of last changes (currently it's used by heap AM only)|
|pd_lower||LocationIndex||2 bytes||Offset to start of free space.|
|pd_upper||LocationIndex||2 bytes||Offset to end of free space.|
|pd_special||LocationIndex||2 bytes||Offset to start of special space.|
|pd_pagesize_version||uint16||2 bytes||Page size and layout version number information.|
All the details may be found in src/include/storage/bufpage.h.
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, pd_pagesize_version, store both the page size and a version indicator. Beginning with PostgreSQL 7.3 the version number is 1; prior releases used version number 0. (The basic page layout and header format has not changed, but the layout of heap tuple headers has.) The page size is basically only present as a cross-check; there is no support for having more than one page size in an installation.
Following the page header are item identifiers (ItemIdData), each requiring four bytes. 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. New item identifiers are allocated as needed from the beginning of the unallocated space. The number of item identifiers present can be determined by looking at pd_lower, which is increased to allocate a new identifier. Because an item identifier is never moved until it is freed, its index may be used on a long-term basis to reference an item, even when the item itself is moved around on the page to compact free space. In fact, every pointer to an item (ItemPointer, also known as CTID) created by PostgreSQL consists of a page number and the index of an item identifier.
The items themselves are stored in space allocated backwards from the end of unallocated space. The exact structure varies depending on what the table is to contain. Tables and sequences both use a structure named HeapTupleHeaderData, described below.
The final section is the "special section" which may contain anything the access method wishes to store. Ordinary tables do not use this at all (indicated by setting pd_special to equal the pagesize).
All table tuples are structured the same way. There is a fixed-size header (occupying 23 bytes on most machines), followed by an optional null bitmap, an optional object ID field, and the user data. The header is detailed in Table 7-3. The actual user data (fields of the tuple) begins at the offset indicated by t_hoff, which must always be a multiple of the MAXALIGN distance for the platform. The null bitmap is only present if the HEAP_HASNULL bit is set in t_infomask. If it is present it begins just after the fixed header and occupies enough bytes to have one bit per data column (that is, t_natts bits altogether). In this list of bits, a 1 bit indicates not-null, a 0 bit is a null. When the bitmap is not present, all columns are assumed not-null. The object ID is only present if the HEAP_HASOID bit is set in t_infomask. If present, it appears just before the t_hoff boundary. Any padding needed to make t_hoff a MAXALIGN multiple will appear between the null bitmap and the object ID. (This in turn ensures that the object ID is suitably aligned.)
Table 7-3. HeapTupleHeaderData Layout
|t_xmin||TransactionId||4 bytes||insert XID stamp|
|t_cmin||CommandId||4 bytes||insert CID stamp (overlays with t_xmax)|
|t_xmax||TransactionId||4 bytes||delete XID stamp|
|t_cmax||CommandId||4 bytes||delete CID stamp (overlays with t_xvac)|
|t_xvac||TransactionId||4 bytes||XID for VACUUM operation moving tuple|
|t_ctid||ItemPointerData||6 bytes||current TID of this or newer tuple|
|t_natts||int16||2 bytes||number of attributes|
|t_infomask||uint16||2 bytes||various flags|
|t_hoff||uint8||1 byte||offset to user data|
All the details may be found in src/include/access/htup.h.
Interpreting the actual data can only be done with information obtained from other tables, mostly pg_attribute. The particular fields are attlen and attalign. There is no way to directly get a particular attribute, except when there are only fixed width fields and no NULLs. All this trickery is wrapped up in the functions heap_getattr, fastgetattr and heap_getsysattr.
To read the data you need to examine each attribute in turn. First check whether the field is NULL according to the null bitmap. If it is, go to the next. Then make sure you have the right alignment. If the field is a fixed width field, then all the bytes are simply placed. If it's a variable length field (attlen == -1) then it's a bit more complicated, using the variable length structure varattrib. Depending on the flags, the data may be either inline, compressed or in another table (TOAST).