sqlite的数据库以单文件的形式存放于磁盘中，比如demo.db文件；
表是以b+树的形式进行组织, 索引是b树组织的；
sqlite_master表是sqlite的数据库系统表，存放所有的用户表的schema；
数据库文件是page的数组，每个page默认大小为1024个字节；
第1个page的头100个字节是数据库文件的头信息（元数据），具体参见表1；
第1个page同时是sqlite_master表的root page；
sqlite_master的其中一个字段存放了其他用户表的root page number；
b树的节点类型类型主要有四种，主要通过page的头信息进行表明，节点头信息参见表2；
每个b树内部是多个cell组成，每个cell的内容见表3；
删除数据并不会立即删除，回收到freelist中，如图1；
单个tuple的长度溢出一个page大小时，采用溢出页的方式，如图2；
表1：Database Header Format
| Offset | Size | Description |
|---|---|---|
| 0 | 16 | The header string: "SQLite format 3\000" |
| 16 | 2 | The database page size in bytes. Must be a power of two between 512 and 32768 inclusive, or the value 1 representing a page size of 65536. |
| 18 | 1 | File format write version. 1 for legacy; 2 for WAL. |
| 19 | 1 | File format read version. 1 for legacy; 2 for WAL. |
| 20 | 1 | Bytes of unused "reserved" space at the end of each page. Usually 0. |
| 21 | 1 | Maximum embedded payload fraction. Must be 64. |
| 22 | 1 | Minimum embedded payload fraction. Must be 32. |
| 23 | 1 | Leaf payload fraction. Must be 32. |
| 24 | 4 | File change counter. |
| 28 | 4 | Size of the database file in pages. The "in-header database size". |
| 32 | 4 | Page number of the first freelist trunk page. |
| 36 | 4 | Total number of freelist pages. |
| 40 | 4 | The schema cookie. |
| 44 | 4 | The schema format number. Supported schema formats are 1, 2, 3, and 4. |
| 48 | 4 | Default page cache size. |
| 52 | 4 | The page number of the largest root b-tree page when in auto-vacuum or incremental-vacuum modes, or zero otherwise. |
| 56 | 4 | The database text encoding. A value of 1 means UTF-8. A value of 2 means UTF-16le. A value of 3 means UTF-16be. |
| 60 | 4 | The "user version" as read and set by the user_version pragma. |
| 64 | 4 | True (non-zero) for incremental-vacuum mode. False (zero) otherwise. |
| 68 | 4 | The "Application ID" set by PRAGMA application_id. |
| 72 | 20 | Reserved for expansion. Must be zero. |
| 92 | 4 | The version-valid-for number. |
| 96 | 4 | SQLITE_VERSION_NUMBER |
表2：B-tree Page Header Format
| Offset | Size | Description |
|---|---|---|
| 0 | 1 | The one-byte flag at offset 0 indicating the b-tree page type. A value of 2 (0x02) means the page is an interior index b-tree page. A value of 5 (0x05) means the page is an interior table b-tree page. A value of 10 (0x0a) means the page is a leaf index b-tree page. A value of 13 (0x0d) means the page is a leaf table b-tree page. Any other value for the b-tree page type is an error. |
| 1 | 2 | The two-byte integer at offset 1 gives the start of the first freeblock on the page, or is zero if there are no freeblocks. |
| 3 | 2 | The two-byte integer at offset 3 gives the number of cells on the page. |
| 5 | 2 | The two-byte integer at offset 5 designates the start of the cell content area. A zero value for this integer is interpreted as 65536. |
| 7 | 1 | The one-byte integer at offset 7 gives the number of fragmented free bytes within the cell content area. |
| 8 | 4 | The four-byte page number at offset 8 is the right-most pointer. This value appears in the header of interior b-tree pages only and is omitted from all other pages. |
表3：B-tree Cell Format
| Datatype | Appears in... | Description | |||
|---|---|---|---|---|---|
| Table Leaf (0x0d) | Table Interior (0x05) | Index Leaf (0x0a) | Index Interior (0x02) | ||
| 4-byte integer | ✔ | ✔ | Page number of left child | ||
| varint | ✔ | ✔ | ✔ | Number of bytes of payload | |
| varint | ✔ | ✔ | Rowid | ||
| byte array | ✔ | ✔ | ✔ | Payload | |
| 4-byte integer | ✔ | ✔ | ✔ | Page number of first overflow page | |
图1：freelist page
图2：溢出页
