File Format Changes in SQLite

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## File Format Changes in SQLite

Every effort is made to keep SQLite fully backwards compatible from one release to the next. Rarely, however, some enhancements or bug fixes may require a change to the underlying file format. When this happens and you must convert the contents of your databases into a portable ASCII representation using the old version of the library then reload the data using the new version of the library.

You can tell if you should reload your databases by comparing the version numbers of the old and new libraries. If the first digit of the version number is different, then a reload of the database will be required. If the second digit changes, newer versions of SQLite will be able to read and write older database files, but older versions of the library may have difficulty reading or writing newer database files. For example, upgrading from version 2.8.14 to 3.0.0 requires a reload. Going from version 3.0.8 to 3.1.0 is backwards compatible but not necessarily forwards compatible.

The following table summarizes the SQLite file format changes that have occurred since version 1.0.0:

> Version Change | Approx. Date | Description Of File Format Change
> --- | --- | ---
> 1.0.32 to 2.0.0 | 2001-Sep-20 | Version 1.0.X of SQLite used the GDBM library as its backend interface to the disk. Beginning in version 2.0.0, GDBM was replaced by a custom B-Tree library written especially for SQLite. The new B-Tree backend is twice as fast as GDBM, supports atomic commits and rollback, and stores an entire database in a single disk file instead using a separate file for each table as GDBM does. The two file formats are not even remotely similar.

> 2.0.8 to 2.1.0

> 2001-Nov-12

> The same basic B-Tree format is used but the details of the index keys were changed in order to provide better query optimization opportunities. Some of the headers were also changed in order to increase the maximum size of a row from 64KB to 24MB.

> This change is an exception to the version number rule described above in that it is neither forwards or backwards compatible. A complete reload of the database is required. This is the only exception.

> 2.1.7 to 2.2.0 | 2001-Dec-21 | Beginning with version 2.2.0, SQLite no longer builds an index for an INTEGER PRIMARY KEY column. Instead, it uses that column as the actual B-Tree key for the main table. Version 2.2.0 and later of the library will automatically detect when it is reading a 2.1.x database and will disable the new INTEGER PRIMARY KEY feature. In other words, version 2.2.x is backwards compatible to version 2.1.x. But version 2.1.x is not forward compatible with version 2.2.x. If you try to open a 2.2.x database with an older 2.1.x library and that database contains an INTEGER PRIMARY KEY, you will likely get a coredump. If the database schema does not contain any INTEGER PRIMARY KEYs, then the version 2.1.x and version 2.2.x database files will be identical and completely interchangeable.
> 2.2.5 to 2.3.0 | 2002-Jan-30 | Beginning with version 2.3.0, SQLite supports some additional syntax (the "ON CONFLICT" clause) in the CREATE TABLE and CREATE INDEX statements that are stored in the SQLITE_MASTER table. If you create a database that contains this new syntax, then try to read that database using version 2.2.5 or earlier, the parser will not understand the new syntax and you will get an error. Otherwise, databases for 2.2.x and 2.3.x are interchangeable.
> 2.3.3 to 2.4.0 | 2002-Mar-10 | Beginning with version 2.4.0, SQLite added support for views. Information about views is stored in the SQLITE_MASTER table. If an older version of SQLite attempts to read a database that contains VIEW information in the SQLITE_MASTER table, the parser will not understand the new syntax and initialization will fail. Also, the way SQLite keeps track of unused disk blocks in the database file changed slightly. If an older version of SQLite attempts to write a database that was previously written by version 2.4.0 or later, then it may leak disk blocks.
> 2.4.12 to 2.5.0 | 2002-Jun-17 | Beginning with version 2.5.0, SQLite added support for triggers. Information about triggers is stored in the SQLITE_MASTER table. If an older version of SQLite attempts to read a database that contains a CREATE TRIGGER in the SQLITE_MASTER table, the parser will not understand the new syntax and initialization will fail.

> 2.5.6 to 2.6.0

> 2002-July-17

> A design flaw in the layout of indices required a file format change to correct. This change appeared in version 2.6.0.

> If you use version 2.6.0 or later of the library to open a database file that was originally created by version 2.5.6 or earlier, an attempt to rebuild the database into the new format will occur automatically. This can take some time for a large database. (Allow 1 or 2 seconds per megabyte of database under Unix - longer under Windows.) This format conversion is irreversible. It is strongly suggested that you make a backup copy of older database files prior to opening them with version 2.6.0 or later of the library, in case there are errors in the format conversion logic.

> Version 2.6.0 or later of the library cannot open read-only database files from version 2.5.6 or earlier, since read-only files cannot be upgraded to the new format.

> 2.6.3 to 2.7.0

> 2002-Aug-13

> Beginning with version 2.7.0, SQLite understands two different datatypes: text and numeric. Text data sorts in memcmp() order. Numeric data sorts in numerical order if it looks like a number, or in memcmp() order if it does not.

> When SQLite version 2.7.0 or later opens a 2.6.3 or earlier database, it assumes all columns of all tables have type "numeric". For 2.7.0 and later databases, columns have type "text" if their datatype string contains the substrings "char" or "clob" or "blob" or "text". Otherwise they are of type "numeric".

> Because "text" columns have a different sort order from numeric, indices on "text" columns occur in a different order for version 2.7.0 and later database. Hence version 2.6.3 and earlier of SQLite will be unable to read a 2.7.0 or later database. But version 2.7.0 and later of SQLite will read earlier databases.

> 2.7.6 to 2.8.0

> 2003-Feb-14

> Version 2.8.0 introduces a change to the format of the rollback journal file. The main database file format is unchanged. Versions 2.7.6 and earlier can read and write 2.8.0 databases and vice versa. Version 2.8.0 can rollback a transaction that was started by version 2.7.6 and earlier. But version 2.7.6 and earlier cannot rollback a transaction started by version 2.8.0 or later.

> The only time this would ever be an issue is when you have a program using version 2.8.0 or later that crashes with an incomplete transaction, then you try to examine the database using version 2.7.6 or earlier. The 2.7.6 code will not be able to read the journal file and thus will not be able to rollback the incomplete transaction to restore the database.

> 2.8.14 to 3.0.0 | 2004-Jun-18 | Version 3.0.0 is a major upgrade for SQLite that incorporates support for UTF-16, BLOBs, and a more compact encoding that results in database files that are typically 25% to 50% smaller. The new file format is very different and is completely incompatible with the version 2 file format.
> 3.0.8 to 3.1.0 | 2005-Jan-21 | Version 3.1.0 adds support for autovacuum mode. Prior versions of SQLite will be able to read an autovacuumed database but will not be able to write it. If autovaccum is disabled (which is the default condition) then databases are fully forwards and backwards compatible.
> 3.1.6 to 3.2.0 | 2005-Mar-19 | Version 3.2.0 adds support for the ALTER TABLE ADD COLUMN command. A database that has been modified by this command can not be read by a version of SQLite prior to 3.1.4. Running VACUUM after the ALTER TABLE restores the database to a format such that it can be read by earlier SQLite versions.

> 3.2.8 to 3.3.0

> 2006-Jan-10

> Version 3.3.0 adds support for descending indices and uses a new encoding for boolean values that requires less disk space. Version 3.3.0 can read and write database files created by prior versions of SQLite. But prior versions of SQLite will not be able to read or write databases created by Version 3.3.0

> If you need backwards and forwards compatibility, you can compile with -DSQLITE_DEFAULT_FILE_FORMAT=1. Or at runtime you can say "PRAGMA legacy_file_format=ON" prior to creating a new database file

> Once a database file is created, its format is fixed. So a database file created by SQLite 3.2.8 and merely modified by version 3.3.0 or later will retain the old format. Except, the VACUUM command recreates the database so running VACUUM on 3.3.0 or later will change the file format to the latest edition.

> 3.3.6 to 3.3.7

> 2006-Aug-12

> The previous file format change has caused so much grief that the default behavior has been changed back to the original file format. This means that DESC option on indices is ignored by default that the more efficient encoding of boolean values is not used. In that way, older versions of SQLite can read and write databases created by newer versions. If the new features are desired, they can be enabled using pragma: "PRAGMA legacy_file_format=OFF".

> To be clear: both old and new file formats continue to be understood and continue to work. But the old file format is used by default instead of the new. This might change again in some future release - we may go back to generating the new file format by default - but probably not until all users have upgraded to a version of SQLite that will understand the new file format. That might take several years.

> 3.4.2 to 3.5.0

> 2007-Sep-3

> The design of the OS interface layer was changed for release 3.5.0. Applications that implemented a custom OS interface will need to be modified in order to upgrade. There are also some subtly different semantics a few obscure APIs. An article is available which describing the changes in detail.

> The on-disk file format is unchanged.

> 3.5.9 to 3.6.0 | 2008-July-16 | There are minor tweaks to the new OS interface layer that was added in version 3.5.0. Applications that implemented a custom OS interface will need to be adjusted. An article is available which describing the changes in detail. The on-disk file format is unchanged.

To perform a database reload, have ready versions of the sqlite command-line utility for both the old and new version of SQLite. Call these two executables "sqlite-old" and "sqlite-new". Suppose the name of your old database is "old.db" and you want to create a new database with the same information named "new.db". The command to do this is as follows:

> sqlite-old old.db .dump | sqlite-new new.db