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Recovering Data From A Corrupt SQLite Database

# 1. Recovering (Some) Data From A Corrupt SQLite Database

SQLite databases are remarkably rebust. Application faults and power failures typically leave the content of the database intact. However, it is possible to corrupt an SQLite database. For example, hardware malfunctions can damage the database file, or a rogue process can open the database and overwrite parts of it.

Given a corrupt database file, it is sometimes desirable to try to salvage as much data from the file as possible. The recovery API is designed to facilitate this.

## 1.1. Limitations

It is sometimes possible to perfectly restore a database that has gone corrupt, but that is the exception. Usually the recovered database will be defective in a number of ways:

- Some content might be permanently deleted and unrecoverable. This can happen, for example, if a rogue process overwrites part of the database file.

- Previously deleted content might reappear. Normally when SQLite does a DELETE operation, it does not actually overwrite the old content but instead remembers that space is available for reuse during the next INSERT. If such deleted content is still in the file when a recovery is attempted, it might be extracted and "resurrected".

- Recovered content might be altered. For example, the value stored in a particular row might change from 48 to 49. Or it might change from an integer into a string or blob. A value that was NULL might become an integer. A string value might become a BLOB. And so forth.

- Constraints may not be valid after recovery. CHECK constraints, FOREIGN KEY constraints, UNIQUE constraints, type constraints on STRICT tables - any of these might be violated in the recovered database.

- Content might be moved from one table into another.

The recovery API does as good of a job as it can at restoring a database, but the results will always be suspect. Sometimes (for example if the corruption is restricted to indexes) the recovery will perfectly restore the database content. However in other cases, the recovery will be imperfect. The impact of this imperfection depends on the application. A database that holds a list of bookmarks is still a list of bookmarks after recovery. A few bookmarks might be missing or added or altered after recovery, but the list is "fuzzy" and imperfect to begin with so adding a bit more uncertainty will not be fatal to the application. But if an accounting database goes corrupt and is subsequently recovered, the books might be out of balance.

It is best to think of the recovery API as a salvage undertaking. Recovery will extract as much usable data as it can from the wreck of the old database, but some parts may be damaged beyond repair and some rework and testing should be performed prior to returning the recovered database to service.

# 2. Recovery Using The ".recover" Command In The CLI

The easiest way to manually recover a corrupt database is using the Command Line Interface or "CLI" for SQLite. The CLI is a program named "sqlite3". Use it to recover a corrupt database file using a command similar to the following:

    sqlite3 corrupt.db .recover >data.sql

This will generate SQL text in the file named "data.sql" that can be used to reconstruct the original database:

    sqlite3 recovered.db <data.sql

The ".recover" option is actually a command that is issued to the CLI. That command can accept arguments. For example, by running:

    sqlite3 corruptdb ".recover --ignore-freelist" >data.sql

Notice that the ".recover" command and its arguments must be contained in quotes. The following options are supported:

> --ignore-freelist

>   Ignore pages of the database that appear to be part of the freelist. Normally the freelist is scanned, and if it contains pages that look like they have content, that content is output. But if the page really is on the freelist, that can mean that previously deleted information is reintroduced into the database.

> --lost-and-found TABLE

>   If content is found during recovery that cannot be associated with a particular table, it is put into the "lost_and_found" table. Use this option to change the name of the "lost_and_found" table to "TABLE".

> --no-rowids

>   If this option is provided, then rowid values that are not also INTEGER PRIMARY KEY values are not extracted from the corrupt database.

# 3. Building The Recovery API Into An Application

## 3.1. Source Code Files

If you want to build the recovery API into your application, you will need to add some source files to your build, above and beyond the usual "sqlite3.c" and "sqlite3.h" source files. You will need:

> sqlite3recover.c | This is the main source file that implements the recovery API.
> sqlite3recover.h | This is the header file that goes with sqlite3recover.h.
> dbdata.c | This file implements two virtual tables name "sqlite_dbdata" and "sqlite_dbptr" that required by sqlite3recover.c.

The two C source file above need to be linked into your application in the same way as "sqlite3.c" is linked in. And the header file needs to be accessible to the compiler when the C files are being compiled.

## 3.2. How To Implement Recovery

These are the basic steps needed to recover content from a corrupt Database:

1. Creates an sqlite3_recover handle by calling either sqlite3_recover_init() or sqlite3_recover_init_sql(). Use sqlite3_recover_init() to store the recovered content in a separate database and use sqlite3_recover_init_sql() to generate SQL text that will reconstruct the database.

2. Make zero or more calls to sqlite3_recover_config() to set options on the new sqlite3_recovery handle.

3. Invoke sqlite3_recover_step() repeatedly until it returns something other than SQLITE_OK. If it returns SQLITE_DONE, then the recovery operation completed without error. If it returns some other non-SQLITE_OK value, then an error has occurred. The sqlite3_recover_run() interface is also available as a convenience wrapper that simply invokes sqlite3_recover_step() repeatedly until it returns something other than SQLITE_DONE.

4. Retrieves any error code and English language error message using the sqlite3_recover_errcode() and sqlite3_recover_errmsg() interfaces, respectively.

5. Invoke sqlite3_recover_finish() to destroy the sqlite3_recover object.

Details of the interface are described in comments in the sqlite3_recover.h header file.

## 3.3. Example Implementations

Examples of how the recovery extension is used by SQLite itself can be seen at the following links:

- https://sqlite.org/src/info/30475c820dc5ab8a8?ln=999,1026

  An example of the recovery extension found in the "fuzzcheck" testing utility in the SQLite tree.

- https://sqlite.org/src/info/84bb08d8762920285f08f1c0?ln=7299,7361

  The code that implements the ".recover" command in the CLI.

This page last modified on 2022-11-04 15:23:18 UTC