sqlite数据库 源码

/* ** 2004 April 6 ** ** The author disclaims copyright to this source code. In place of ** a legal notice, here is a blessing: ** ** May you do good and not evil. ** May you find forgiveness for yourself and forgive others. ** May you share freely, never taking more than you give. ** ************************************************************************* ** $Id: btree.c,v 1.326 2006/08/08 13:51:43 drh Exp $ ** ** This file implements a external (disk-based) database using BTrees. ** For a detailed discussion of BTrees, refer to ** ** Donald E. Knuth, THE ART OF COMPUTER PROGRAMMING, Volume 3: ** "Sorting And Searching", pages 473-480. Addison-Wesley ** Publishing Company, Reading, Massachusetts. ** ** The basic idea is that each page of the file contains N database ** entries and N+1 pointers to subpages. ** ** ---------------------------------------------------------------- ** | Ptr(0) | Key(0) | Ptr(1) | Key(1) | ... | Key(N) | Ptr(N+1) | ** ---------------------------------------------------------------- ** ** All of the keys on the page that Ptr(0) points to have values less ** than Key(0). All of the keys on page Ptr(1) and its subpages have ** values greater than Key(0) and less than Key(1). All of the keys ** on Ptr(N+1) and its subpages have values greater than Key(N). And ** so forth. ** ** Finding a particular key requires reading O(log(M)) pages from the ** disk where M is the number of entries in the tree. ** ** In this implementation, a single file can hold one or more separate ** BTrees. Each BTree is identified by the index of its root page. The ** key and data for any entry are combined to form the "payload". A ** fixed amount of payload can be carried directly on the database ** page. If the payload is larger than the preset amount then surplus ** bytes are stored on overflow pages. The payload for an entry ** and the preceding pointer are combined to form a "Cell". Each ** page has a small header which contains the Ptr(N+1) pointer and other ** information such as the size of key and data. ** ** FORMAT DETAILS ** ** The file is divided into pages. The first page is called page 1, ** the second is page 2, and so forth. A page number of zero indicates ** "no such page". The page size can be anything between 512 and 65536. ** Each page can be either a btree page, a freelist page or an overflow ** page. ** ** The first page is always a btree page. The first 100 bytes of the first ** page contain a special header (the "file header") that describes the file. ** The format of the file header is as follows: ** ** OFFSET SIZE DESCRIPTION ** 0 16 Header string: "SQLite format 3\000" ** 16 2 Page size in bytes. ** 18 1 File format write version ** 19 1 File format read version ** 20 1 Bytes of unused space at the end of each page ** 21 1 Max embedded payload fraction ** 22 1 Min embedded payload fraction ** 23 1 Min leaf payload fraction ** 24 4 File change counter ** 28 4 Reserved for future use ** 32 4 First freelist page ** 36 4 Number of freelist pages in the file ** 40 60 15 4-byte meta values passed to higher layers ** ** All of the integer values are big-endian (most significant byte first). ** ** The file change counter is incremented when the database is changed more ** than once within the same second. This counter, together with the ** modification time of the file, allows other processes to know ** when the file has changed and thus when they need to flush their ** cache. ** ** The max embedded payload fraction is the amount of the total usable ** space in a page that can be consumed by a single cell for standard ** B-tree (non-LEAFDATA) tables. A value of 255 means 100%. The default ** is to limit the maximum cell size so that at least 4 cells will fit ** on one page. Thus the default max embedded payload fraction is 64. ** ** If the payload for a cell is larger than the max payload, then extra ** payload is spilled to overflow pages. Once an overflow page is allocated, ** as many bytes as possible are moved into the overflow pages without letting ** the cell size drop below the min embedded payload fraction. ** ** The min leaf payload fraction is like the min embedded payload fraction ** except that it applies to leaf nodes in a LEAFDATA tree. The maximum ** payload fraction for a LEAFDATA tree is always 100% (or 255) and it ** not specified in the header. ** ** Each btree pages is divided into three sections: The header, the ** cell pointer array, and the cell area area. Page 1 also has a 100-byte ** file header that occurs before the page header. ** ** |----------------| ** | file header | 100 bytes. Page 1 only. ** |----------------| ** | page header | 8 bytes for leaves. 12 bytes for interior nodes ** |----------------| ** | cell pointer | | 2 bytes per cell. Sorted order. ** | array | | Grows downward ** | | v ** |----------------| ** | unallocated | ** | space | ** |----------------| ^ Grows upwards ** | cell content | | Arbitrary order interspersed with freeblocks. ** | area | | and free space fragments. ** |----------------| ** ** The page headers looks like this: ** ** OFFSET SIZE DESCRIPTION ** 0 1 Flags. 1: intkey, 2: zerodata, 4: leafdata, 8: leaf ** 1 2 byte offset to the first freeblock ** 3 2 number of cells on this page ** 5 2 first byte of the cell content area ** 7 1 number of fragmented free bytes ** 8 4 Right child (the Ptr(N+1) value). Omitted on leaves. ** ** The flags define the format of this btree page. The leaf flag means that ** this page has no children. The zerodata flag means that this page carries ** only keys and no data. The intkey flag means that the key is a integer ** which is stored in the key size entry of the cell header rather than in ** the payload area. ** ** The cell pointer array begins on the first byte after the page header. ** The cell pointer array contains zero or more 2-byte numbers which are ** offsets from the beginning of the page to the cell content in the cell ** content area. The cell pointers occur in sorted order. The system strives ** to keep free space after the last cell pointer so that new cells can ** be easily added without having to defragment the page. ** ** Cell content is stored at the very end of the page and grows toward the ** beginning of the page. ** ** Unused space within the cell content area is collected into a linked list of ** freeblocks. Each freeblock is at least 4 bytes in size. The byte offset ** to the first freeblock is given in the header. Freeblocks occur in ** increasing order. Because a freeblock must be at least 4 bytes in size, ** any group of 3 or fewer unused bytes in the cell content area cannot ** exist on the freeblock chain. A group of 3 or fewer free bytes is called ** a fragment. The total number of bytes in all fragments is recorded. ** in the page header at offset 7. ** ** SIZE DESCRIPTION ** 2 Byte offset of the next freeblock ** 2 Bytes in this freeblock ** ** Cells are of variable length. Cells are stored in the cell content area at ** the end of the page. Pointers to the cells are in the cell pointer array ** that immediately follows the page header. Cells is not necessarily ** contiguous or in order, but cell pointers are contiguous and in order. ** ** Cell content makes use of variable length integers. A variable ** length integer is 1 to 9 bytes where the lower 7 bits of each ** byte are used. The integer consists of all bytes that have bit 8 set and ** the first byte with bit 8 clear.