This chapter provides an overview of the physical storage format used by PostgreSQL databases. This section describes the storage format at the level of files and directories. All the data needed for a database cluster is stored within the cluster's data directory, commonly referred to as PGDATA (after the name of the environment variable that can be used to define it). A common location for PGDATA is /var/lib/pgsql/data. Multiple clusters, managed by different server instances, can exist on the same machine. The PGDATA directory contains several subdirectories and control files, as shown in . In addition to these required items, the cluster configuration files postgresql.conf, pg_hba.conf, and pg_ident.conf are traditionally stored in PGDATA (although in PostgreSQL 8.0 and later, it is possible to keep them elsewhere). Item Description PG_VERSION A file containing the major version number of PostgreSQL base Subdirectory containing per-database subdirectories global Subdirectory containing cluster-wide tables, such as pg_database pg_clog Subdirectory containing transaction commit status data pg_multixact Subdirectory containing multitransaction status data (used for shared row locks) pg_subtrans Subdirectory containing subtransaction status data pg_tblspc Subdirectory containing symbolic links to tablespaces pg_twophase Subdirectory containing state files for prepared transactions pg_xlog Subdirectory containing WAL (Write Ahead Log) files postmaster.opts A file recording the command-line options the server was last started with postmaster.pid A lock file recording the current server PID and shared memory segment ID (not present after server shutdown) For each database in the cluster there is a subdirectory within PGDATA/base, named after the database's OID in pg_database. This subdirectory is the default location for the database's files; in particular, its system catalogs are stored there. Each table and index is stored in a separate file, named after the table or index's filenode number, which can be found in pg_class.relfilenode. Note that while a table's filenode often matches its OID, this is not necessarily the case; some operations, like TRUNCATE, REINDEX, CLUSTER and some forms of ALTER TABLE, can change the filenode while preserving the OID. Avoid assuming that filenode and table OID are the same. When a table or index exceeds 1 GB, it is divided into gigabyte-sized segments. The first segment's file name is the same as the filenode; subsequent segments are named filenode.1, filenode.2, etc. This arrangement avoids problems on platforms that have file size limitations. The contents of tables and indexes are discussed further in . A table that has columns with potentially large entries will have an associated TOAST table, which is used for out-of-line storage of field values that are too large to keep in the table rows proper. pg_class.reltoastrelid links from a table to its TOAST table, if any. See for more information. Tablespaces make the scenario more complicated. Each user-defined tablespace has a symbolic link inside the PGDATA/pg_tblspc directory, which points to the physical tablespace directory (as specified in its CREATE TABLESPACE command). The symbolic link is named after the tablespace's OID. Inside the physical tablespace directory there is a subdirectory for each database that has elements in the tablespace, named after the database's OID. Tables within that directory follow the filenode naming scheme. The pg_default tablespace is not accessed through pg_tblspc, but corresponds to PGDATA/base. Similarly, the pg_global tablespace is not accessed through pg_tblspc, but corresponds to PGDATA/global. Temporary files (for operations such as sorting more data than can fit in memory) are created within PGDATA/base/pgsql_tmp, or within a pgsql_tmp subdirectory of a tablespace directory if a tablespace other than pg_default is specified for them. The name of a temporary file has the form pgsql_tmpPPP.NNN, where PPP is the PID of the owning backend and NNN distinguishes different files of that backend. TOAST sliced breadTOAST This section provides an overview of TOAST (The Oversized-Attribute Storage Technique). PostgreSQL uses a fixed page size (commonly 8 kB), and does not allow tuples to span multiple pages. Therefore, it is not possible to store very large field values directly. To overcome this limitation, large field values are compressed and/or broken up into multiple physical rows. This happens transparently to the user, with only small impact on most of the backend code. The technique is affectionately known as TOAST (or the best thing since sliced bread). Only certain data types support TOAST — there is no need to impose the overhead on data types that cannot produce large field values. To support TOAST, a data type must have a variable-length (varlena) representation, in which the first 32-bit word of any stored value contains the total length of the value in bytes (including itself). TOAST does not constrain the rest of the representation. All the C-level functions supporting a TOAST-able data type must be careful to handle TOASTed input values. (This is normally done by invoking PG_DETOAST_DATUM before doing anything with an input value, but in some cases more efficient approaches are possible.) TOAST usurps two bits of the varlena length word (the high-order bits on big-endian machines, the low-order bits on little-endian machines), thereby limiting the logical size of any value of a TOAST-able data type to 1 GB (230 - 1 bytes). When both bits are zero, the value is an ordinary un-TOASTed value of the data type, and the remaining bits of the length word give the total datum size (including length word) in bytes. When the highest-order or lowest-order bit is set, the value has only a single-byte header instead of the normal four-byte header, and the remaining bits give the total datum size (including length byte) in bytes. As a special case, if the remaining bits are all zero (which would be impossible for a self-inclusive length), the value is a pointer to out-of-line data stored in a separate TOAST table. (The size of a TOAST pointer is given in the second byte of the datum.) Values with single-byte headers aren't aligned on any particular boundary, either. Lastly, when the highest-order or lowest-order bit is clear but the adjacent bit is set, the content of the datum has been compressed and must be decompressed before use. In this case the remaining bits of the length word give the total size of the compressed datum, not the original data. Note that compression is also possible for out-of-line data but the varlena header does not tell whether it has occurred — the content of the TOAST pointer tells that, instead. If any of the columns of a table are TOAST-able, the table will have an associated TOAST table, whose OID is stored in the table's pg_class.reltoastrelid entry. Out-of-line TOASTed values are kept in the TOAST table, as described in more detail below. The compression technique used is a fairly simple and very fast member of the LZ family of compression techniques. See src/backend/utils/adt/pg_lzcompress.c for the details. Out-of-line values are divided (after compression if used) into chunks of at most TOAST_MAX_CHUNK_SIZE bytes (by default this value is chosen so that four chunk rows will fit on a page, making it about 2000 bytes). Each chunk is stored as a separate row in the TOAST table for the owning table. Every TOAST table has the columns chunk_id (an OID identifying the particular TOASTed value), chunk_seq (a sequence number for the chunk within its value), and chunk_data (the actual data of the chunk). A unique index on chunk_id and chunk_seq provides fast retrieval of the values. A pointer datum representing an out-of-line TOASTed value therefore needs to store the OID of the TOAST table in which to look and the OID of the specific value (its chunk_id). For convenience, pointer datums also store the logical datum size (original uncompressed data length) and actual stored size (different if compression was applied). Allowing for the varlena header bytes, the total size of a TOAST pointer datum is therefore 18 bytes regardless of the actual size of the represented value. The TOAST code is triggered only when a row value to be stored in a table is wider than TOAST_TUPLE_THRESHOLD bytes (normally 2 kB). The TOAST code will compress and/or move field values out-of-line until the row value is shorter than TOAST_TUPLE_TARGET bytes (also normally 2 kB) or no more gains can be had. During an UPDATE operation, values of unchanged fields are normally preserved as-is; so an UPDATE of a row with out-of-line values incurs no TOAST costs if none of the out-of-line values change. The TOAST code recognizes four different strategies for storing TOAST-able columns: PLAIN prevents either compression or out-of-line storage; furthermore it disables use of single-byte headers for varlena types. This is the only possible strategy for columns of non-TOAST-able data types. EXTENDED allows both compression and out-of-line storage. This is the default for most TOAST-able data types. Compression will be attempted first, then out-of-line storage if the row is still too big. EXTERNAL allows out-of-line storage but not compression. Use of EXTERNAL will make substring operations on wide text and bytea columns faster (at the penalty of increased storage space) because these operations are optimized to fetch only the required parts of the out-of-line value when it is not compressed. MAIN allows compression but not out-of-line storage. (Actually, out-of-line storage will still be performed for such columns, but only as a last resort when there is no other way to make the row small enough.) Each TOAST-able data type specifies a default strategy for columns of that data type, but the strategy for a given table column can be altered with ALTER TABLE SET STORAGE. This scheme has a number of advantages compared to a more straightforward approach such as allowing row values to span pages. Assuming that queries are usually qualified by comparisons against relatively small key values, most of the work of the executor will be done using the main row entry. The big values of TOASTed attributes will only be pulled out (if selected at all) at the time the result set is sent to the client. Thus, the main table is much smaller and more of its rows fit in the shared buffer cache than would be the case without any out-of-line storage. Sort sets shrink also, and sorts will more often be done entirely in memory. A little test showed that a table containing typical HTML pages and their URLs was stored in about half of the raw data size including the TOAST table, and that the main table contained only about 10% of the entire data (the URLs and some small HTML pages). There was no run time difference compared to an un-TOASTed comparison table, in which all the HTML pages were cut down to 7 kB to fit. This section provides an overview of the page format used within PostgreSQL tables and indexes. Actually, index access methods need not use this page format. All the existing index methods do use this basic format, but the data kept on index metapages usually doesn't follow the item layout rules. Sequences and TOAST tables 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 row; in an index, an item is an index entry. Every table and index is stored as an array of pages of a fixed size (usually 8 kB, although a different page size can be selected when compiling the server). In a table, all the pages are logically equivalent, so a particular item (row) can be stored in any page. In indexes, the first page is generally reserved as a metapage holding control information, and there can be different types of pages within the index, depending on the index access method. shows the overall layout of a page. There are five parts to each page. Page Layout Item Description PageHeaderData 24 bytes long. Contains general information about the page, including free space pointers. ItemPointerData Array of (offset,length) pairs pointing to the actual items. 4 bytes per item. Free space The unallocated space. New item pointers are allocated from the start of this area, new items 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 24 bytes of each page consists of a page header (PageHeaderData). Its format is detailed in . The first two fields track the most recent WAL entry related to this page. Next is a 2-byte field containing flag bits. This is followed by three 2-byte integer fields (pd_lower, pd_upper, and pd_special). These contain byte offsets from the page start to the start of unallocated space, to the end of unallocated space, and to the start of the special space. The next 2 bytes of the page header, pd_pagesize_version, store both the page size and a version indicator. Beginning with PostgreSQL 8.3 the version number is 4; PostgreSQL 8.1 and 8.2 used version number 3; PostgreSQL 8.0 used version number 2; PostgreSQL 7.3 and 7.4 used version number 1; prior releases used version number 0. (The basic page layout and header format has not changed in most of these versions, but the layout of heap row 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. The last field is a hint that shows whether pruning the page is likely to be profitable: it tracks the oldest un-pruned XMAX on the page. PageHeaderData Layout Field Type Length Description pd_lsn XLogRecPtr 8 bytes LSN: next byte after last byte of xlog record for last change to this page pd_tli uint16 2 bytes TimeLineID of last change (only its lowest 16 bits) pd_flags uint16 2 bytes Flag bits 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 pd_prune_xid TransactionId 4 bytes Oldest unpruned XMAX on page, or zero if none All the details can be found in src/include/storage/bufpage.h. 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 few 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 can 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 can contain anything the access method wishes to store. For example, b-tree indexes store links to the page's left and right siblings, as well as some other data relevant to the index structure. Ordinary tables do not use a special section at all (indicated by setting pd_special to equal the page size). All table rows are structured in 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 . The actual user data (columns of the row) 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.) HeapTupleHeaderData Layout Field Type Length Description t_xmin TransactionId 4 bytes insert XID stamp t_xmax TransactionId 4 bytes delete XID stamp t_cid CommandId 4 bytes insert and/or delete CID stamp (overlays with t_xvac) t_xvac TransactionId 4 bytes XID for VACUUM operation moving a row version t_ctid ItemPointerData 6 bytes current TID of this or newer row version t_infomask2 int16 2 bytes number of attributes, plus various flag bits t_infomask uint16 2 bytes various flag bits t_hoff uint8 1 byte offset to user data All the details can 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 key values needed to identify field locations are attlen and attalign. There is no way to directly get a particular attribute, except when there are only fixed width fields and no null values. 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. All variable-length datatypes share the common header structure struct varlena, which includes the total length of the stored value and some flag bits. Depending on the flags, the data can be either inline or in a TOAST table; it might be compressed, too (see ).