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SQLite's Use Of Temporary Disk Files

1.0 Introduction

On of the distinctive features of SQLite is that a database consists of a single disk file. This simplifies the use of SQLite since moving or backing up a database is a simple as copying a single file. It also makes SQLite appropriate for use as an application file format. But while a complete database is held in a single disk file, SQLite does make use of many temporary files during the course of processing a database.

This article describes the various temporary files that SQLite creates and uses. It describes when the files are created, when they are deleted, what they are used for, why they are important, and how to avoid them on systems where creating temporary files is expensive.

The manner in which SQLite uses temporary files is not considered part of the contract that SQLite makes with applications. The information in this document is a correct description of how SQLite operates at the time that this document was written or last updated. But there is no guarantee that future versions of SQLite will use temporary files in the same way. New kinds of temporary files might be employed and some of the current temporary file uses might be discontinued in future releases of SQLite.

2.0 Seven Kinds Of Temporary Files

SQLite currently uses seven distinct types of temporary files:

Rollback journals
Master journals
Statement journals
TEMP databases
Materializations of views and subqueries
Transient indices
Transient databases used by VACUUM

Additional information about each of these temporary file types is in the sequel.

2.1 Rollback Journals

A rollback journal is a temporary file used to implement atomic commit and rollback capabilities in SQLite. (For a detailed discussion of how this works, see the separate document titled Atomic Commit In SQLite.) The rollback journal is always located in the same directory as the database file and has the same name as the database file except with the 8 characters "-journal" appended. The rollback journal is usually created when a transaction is first started and is usually deleted when a transaction commits or rolls back. The rollback journal file is essential for implementing the atomic commit and rollback capabilities of SQLite. Without a rollback journal, SQLite would be unable to rollback an incomplete transaction, and if a crash or power loss occurred in the middle of a transaction the entire database would likely go corrupt without a rollback journal.

The rollback journal is usually created and destroyed at the start and end of a transaction, respectively. But there are exceptions to this rule.

If a crash or power loss occurs in the middle of a transaction, then the rollback journal file is left on disk. The next time another application attempts to open the database file, it notices the presence of the abandoned rollback journal (we call it a "hot journal" in this circumstance) and uses the information in the journal to restore the database to its state prior to the start of the incomplete transaction. This is how SQLite implements atomic commit.

If an application puts SQLite in exclusive locking mode using the pragma:

PRAGMA locking_mode=EXCLUSIVE;

SQLite creates a new rollback journal at the start of the first transaction within an exclusive locking mode session. But at the conclusion of the transaction, it does not delete the rollback journal. The rollback journal might be truncated, or its header might be zeroed (depending on what version of SQLite you are using) but the rollback journal is not deleted. The rollback journal is not deleted until exclusive access mode is exited.

Rollback journal creation and deletion is also changed by the journal_mode pragma. The default journaling mode is DELETE, which is the default behavior of deleting the rollback journal file at the end of each transaction, as described above. The PERSIST journal mode foregoes the deletion of the journal file and instead overwrites the rollback journal header with zeros, which prevents other processes from rolling back the journal and thus has the same effect as deleting the journal file, though without the expense of actually removing the file from disk. In other words, journal mode PERSIST exhibits the same behavior as is seen in EXCLUSIVE locking mode. The OFF journal mode causes SQLite to forego creating a rollback journal in the first place. The OFF journal mode disables the atomic commit and rollback capabilities of SQLite. The ROLLBACK command is not available when OFF journal mode is set. And if a crash or power loss occurs in the middle of a transaction that uses the OFF journal mode, no recovery is possible and the database file will likely go corrupt.

2.2 Master Journal Files

The master journal file is used as part of the atomic commit process when a single transaction makes changes to multiple databases that have been added to a single database connection using the ATTACH statement. The master journal file is always located in the same directory as the main database file (the main database file is the database that is identified in the original sqlite3_open(), sqlite3_open16(), or sqlite3_open_v2() call that created the database connection) with a randomized suffix. The master journal file contains the names of all of the various attached auxiliary databases that were changed during the transaction. The multi-database transaction commits when the master journal file is deleted. See the documentation titled Atomic Commit In SQLite for additional detail.

The master journal file is only created in cases where a single database connection is talking with two or more databases files as a result of using ATTACH to connection to auxiliary databases, and where a single transaction modifies more than one of those database files. Without the master journal, the transaction commit on a multi-database transaction would be atomic for each database individually, but it would not be atomic across all databases. In other words, if the commit were interrupted in the middle by a crash or power loss, then the changes to one of the databases might complete while the changes to another database might roll back. The master journal causes all changes in all databases to either rollback or commit together.

2.3 Statement Journal Files

A statement journal file is used to rollback partial results of a single statement within a larger transaction. For example, suppose an UPDATE statement will attempt to modify 100 rows in the database. But after modifying the first 50 rows, the UPDATE hits a constraint violation which should block the entire statement. The statement journal is used to undo the first 50 row changes so that the database is restored to the state it was in at the start of the statement.

A statement journal is only created for an UPDATE or INSERT statement that might change multiple rows of a database and which might hit a constraint or a RAISE exception within a trigger and thus need to undo partial results. If the UPDATE or INSERT is not contained within BEGIN...COMMIT and if there are no other active statements on the same database connection then no statement journal is created since the ordinary rollback journal can be used instead. The statement journal is also omitted if an alternative conflict resolution algorithm is used. For example:

UPDATE OR FAIL ...
UPDATE OR IGNORE ...
UPDATE OR REPLACE ...
INSERT OR FAIL ...
INSERT OR IGNORE ...
INSERT OR REPLACE ...
REPLACE INTO ....

The statement journal is given a randomized name, not necessarily in the same directory as the main database, and is automatically deleted at the conclusion of the transaction. The size of the statement journal is proportional to the size of the change implemented by the UPDATE or INSERT statement that caused the statement journal to be created.

2.4 TEMP Databases

Tables created using the "CREATE TEMP TABLE" syntax are only visible to the database connection in which the "CREATE TEMP TABLE" statement is originally evaluated. These TEMP tables, together with any associated indices, triggers, and views, are collectively stored in a separate temporary database file that is created as soon as the first "CREATE TEMP TABLE" statement is seen. This separate temporary database file also has an associated rollback journal. The temporary database file used to store TEMP tables is deleted automatically when the database connection is closed using sqlite3_close().

The TEMP database file is very similar to auxiliary database files added using the ATTACH statement, though with a few special properties. The TEMP database is always automatically deleted when the database connection is closed. The TEMP database always uses the synchronous=OFF and journal_mode=PERSIST PRAGMA settings. And, the TEMP database cannot be used with DETACH nor can another process ATTACH the TEMP database.

The temporary files associated with the TEMP database and its rollback journal are only created if the application makes use of the "CREATE TEMP TABLE" statement.

2.5 Materializations Of Views And Subqueries

Queries that contain subqueries must sometime evaluate the subqueries separately and store the results in a temporary table, then use the content of the temporary table to evaluate the outer query. We call this "materializing" the subquery. The query optimizer in SQLite attempts to avoid materializing, but sometimes it is not easily avoidable. The temporary tables created by materialization are each stored in their own separate temporary file, which is automatically deleted at the conclusion of the query. The size of these temporary tables depends on the amount of data in the materialization of the subquery, of course.

A subquery on the right-hand side of IN operator must often be materialized. For example:

SELECT * FROM ex1 WHERE ex1.a IN (SELECT b FROM ex2);

In the query above, the subquery "SELECT b FROM ex2" is evaluated and its results are stored in a temporary table (actually a temporary index) that allows one to determine whether or not a value ex2.b exists using a simple binary search. Once this table is constructed, the outer query is run and for each prospective result row a check is made to see if ex1.a is contained within the temporary table. The row is output only if the check is true.

To avoid creating the temporary table, the query might be rewritten as follows:

SELECT * FROM ex1 WHERE EXISTS(SELECT 1 FROM ex2 WHERE ex2.b=ex1.a);

Recent versions of SQLite (version 3.5.4 and later) will do this rewrite automatically if an index exists on the column ex2.b.

If the right-hand side of an IN operator can be list of values as in the following:

SELECT * FROM ex1 WHERE a IN (1,2,3);

List values on the right-hand side of IN are treated as a subquery that must be materialized. In other words, the previous statement acts as if it were:

SELECT * FROM ex1 WHERE a IN (SELECT 1 UNION ALL
                              SELECT 2 UNION ALL
                              SELECT 3);

A temporary index is always used to hold the values of the right-hand side of an IN operator when that right-hand side is a list of values.

Subqueries might also need to be materialized when they appear in the FROM clause of a SELECT statement. For example:

SELECT * FROM ex1 JOIN (SELECT b FROM ex2) AS t ON t.b=ex1.a;

Depending on the query, SQLite might need to materialize the "(SELECT b FROM ex2)" subquery into a temporary table, then perform the join between ex1 and the temporary table. The query optimizer tries to avoid this by "flattening" the query. In the previous example the query can be flattened, and SQLite will automatically transform the query into

SELECT ex1.*, ex2.b FROM ex1 JOIN ex2 ON ex2.b=ex1.a;

More complex queries may or may not be able to employ query flattening to avoid the temporary table. Whether or not the query can be flattened depends on such factors as whether or not the subquery or outer query contain aggregate functions, ORDER BY or GROUP BY clauses, LIMIT clauses, and so forth. The rules for when a query and cannot be flattened are very complex and are beyond the scope of this document.

2.6 Transient Indices

SQLite may make use of transient indices to implement SQL language features such as:

An ORDER BY or GROUP BY clause
The DISTINCT keyword in an aggregate query
Compound SELECT statements joined by UNION, EXCEPT, or INTERSECT

Each transient index is stored in its own temporary file. The temporary file for a transient index is automatically deleted at the end of the statement that uses it.

SQLite strives to implement ORDER BY clauses using a preexisting index. If an appropriate index already exists, SQLite will walk the index, rather than the underlying table, to extract the requested information, and thus cause the rows to come out in the desired order. But if SQLite cannot find an appropriate index it will evaluate the query and store each row in a transient index whose data is the row data and whose key is the ORDER BY terms. After the query is evaluated, SQLite goes back and walks the transient index from beginning to end in order to output the rows in the desired order.

SQLite implements GROUP BY by ordering the output rows in the order suggested by the GROUP BY terms. Each output row is compared to the previous to see if it starts a new "group". The ordering by GROUP BY terms is done in exactly the same way as the ordering by ORDER BY terms. A preexisting index is used if possible, but if no suitable index is available, a transient index is created.

The previous two paragraphs describe the implementation of SQLite as of version 3.5.8. There are known problems with this approach for very large results sets - result sets that are larger than the available disk cache. Future versions of SQLite will likely address this deficiency by completely reworking the sort algorithm for cases when no suitable preexisting sort index is available. The new sort algorithm will also use temporary files, but not in the same way as the current implementation, the temporary files for the new implementation will probably not be index files.

The DISTINCT keyword on an aggregate query is implemented by creating a transient index in a temporary file and storing each result row in that index. As new result rows are computed a check is made to see if they already exist in the transient index and if they do the new result row is discarded.

The UNION operator for compound queries is implemented by creating a transient index in a temporary file and storing the results of the left and right subquery in the transient index, discarding duplicates. After both subqueries have been evaluated, the transient index is walked from beginning to end to generate the final output.

The EXCEPT operator for compound queries is implemented by creating a transient index in a temporary file, storing the results of the left subquery in this transient index, then removing the result from right subquery from the transient index, and finally walking the index from beginning to end to obtain the final output.

The INTERSECT operator for compound queries is implemented by creating two separate transient indices, each in a separate temporary file. The left and right subqueries are evaluated each into a separate transient index. Then the two indices are walked together and entries that appear in both indices are output.

Note that the UNION ALL operator for compound queries does not use transient indices by itself (though of course the right and left subqueries of the UNION ALL might use transient indices depending on how they are composed.)

2.7 Transient Database Used By VACUUM

The VACUUM command works by creating a temporary file and then rebuilding the entire database into that temporary file. Then the content of the temporary file is copied back into the original database file and the temporary file is deleted.

The temporary file created by the VACUUM command exists only for the duration of the command itself. The size of the temporary file will be no larger than the original database.

3.0 The SQLITE_TEMP_STORE Compile-Time Parameter and Pragma

The rollback journal, master journal, and statement journal files are always written to disk. But the other kinds of temporary files might be stored in memory only and never written to disk. Whether or not temporary files other than the rollback, master, and statement journals are written to disk or stored only in memory depends on the SQLITE_TEMP_STORE compile-time parameter, the temp_store pragma, and on the size of the temporary file.

The SQLITE_TEMP_STORE compile-time parameter is a #define whose value is an integer between 0 and 3, inclusive. The meaning of the SQLITE_TEMP_STORE compile-time parameter is as follows:

Temporary files are always stored on disk regardless of the setting of the temp_store pragma.
Temporary files are stored on disk by default but this can be overridden by the temp_store pragma.
Temporary files are stored in memory by default but this can be overridden by the temp_store pragma.
Temporary files are always stored in memory regardless of the setting of the temp_store pragma.

The default value of the SQLITE_TEMP_STORE compile-time parameter is 1, which means to store temporary files on disk but provide the option of overriding the behavior using the temp_store pragma.

The temp_store pragma has an integer value which also influences the decision of where to store temporary files. The values of the temp_store pragma have the following meanings:

Use either disk or memory storage for temporary files as determined by the SQLITE_TEMP_STORE compile-time parameter.
If the SQLITE_TEMP_STORE compile-time parameter specifies memory storage for temporary files, then override that decision and use disk storage instead. Otherwise follow the recommendation of the SQLITE_TEMP_STORE compile-time parameter.
If the SQLITE_TEMP_STORE compile-time parameter specifies disk storage for temporary files, then override that decision and use memory storage instead. Otherwise follow the recommendation of the SQLITE_TEMP_STORE compile-time parameter.

The default setting for the temp_store pragma is 0, which means to following the recommendation of SQLITE_TEMP_STORE compile-time parameter.

To reiterate, the SQLITE_TEMP_STORE compile-time parameter and the temp_store pragma only influence the temporary files other than the rollback journal and the master journal. The rollback journal and the master journal are always written to disk regardless of the settings of the SQLITE_TEMP_STORE compile-time parameter and the temp_store pragma.

4.0 Other Temporary File Optimizations

SQLite uses a page cache of recently read and written database pages. This page cache is used not just for the main database file but also for transient indices and tables stored in temporary files. If SQLite needs to use a temporary index or table and the SQLITE_TEMP_STORE compile-time parameter and the temp_store pragma are set to store temporary tables and index on disk, the information is still initially stored in memory in the page cache. The temporary file is not opened and the information is not truly written to disk until the page cache is full.

This means that for many common cases where the temporary tables and indices are small (small enough to fit into the page cache) no temporary files are created and no disk I/O occurs. Only when the temporary data becomes too large to fit in RAM does the information spill to disk.

Each temporary table and index is given its own page cache which can store a maximum number of database pages determined by the SQLITE_DEFAULT_TEMP_CACHE_SIZE compile-time parameter. (The default value is 500 pages.) The maximum number of database pages in the page cache is the same for every temporary table and index. The value cannot be changed at run-time or on a per-table or per-index basis. Each temporary file gets its own private page cache with its own SQLITE_DEFAULT_TEMP_CACHE_SIZE page limit.