Implementation Limits For SQLite

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## Limits In SQLite

"Limits" in the context of this article means sizes or quantities that can not be exceeded. We are concerned with things like the maximum number of bytes in a BLOB or the maximum number of columns in a table.

SQLite was originally designed with a policy of avoiding arbitrary limits. Of course, every program that runs on a machine with finite memory and disk space has limits of some kind. But in SQLite, those limits were not well defined. The policy was that if it would fit in memory and you could count it with a 32-bit integer, then it should work.

Unfortunately, the no-limits policy has been shown to create problems. Because the upper bounds were not well defined, they were not tested, and bugs (including possible security exploits) were often found when pushing SQLite to extremes. For this reason, newer versions of SQLite have well-defined limits and those limits are tested as part of the test suite.

This article defines what the limits of SQLite are and how they can be customized for specific applications. The default settings for limits are normally quite large and adequate for almost every application. Some applications may want to increase a limit here or there, but we expect such needs to be rare. More commonly, an application might want to recompile SQLite with much lower limits to avoid excess resource utilization in the event of bug in higher-level SQL statement generators or to help thwart attackers who inject malicious SQL statements.

Some limits can be changed at run-time on a per-connection basis using the sqlite3_limit() interface with one of the limit categories defined for that interface. Run-time limits are designed for applications that have multiple databases, some of which are for internal use only and others which can be influenced or controlled by potentially hostile external agents. For example, a web browser application might use an internal database to track historical page views but have one or more separate databases that are created and controlled by javascript applications that are downloaded from the internet. The sqlite3_limit() interface allows internal databases managed by trusted code to be unconstrained while simultaneously placing tight limitations on databases created or controlled by untrusted external code in order to help prevent a denial of service attack.

1. Maximum length of a string or BLOB

   The maximum number of bytes in a string or BLOB in SQLite is defined by the preprocessor macro SQLITE_MAX_LENGTH. The default value of this macro is 1 billion (1 thousand million or 1,000,000,000). You can raise or lower this value at compile-time using a command-line option like this:

   > -DSQLITE_MAX_LENGTH=123456789

   The current implementation will only support a string or BLOB length up to 231-1 or 2147483647. And some built-in functions such as hex() might fail well before that point. In security-sensitive applications it is best not to try to increase the maximum string and blob length. In fact, you might do well to lower the maximum string and blob length to something more in the range of a few million if that is possible.

   During part of SQLite's INSERT and SELECT processing, the complete content of each row in the database is encoded as a single BLOB. So the SQLITE_MAX_LENGTH parameter also determines the maximum number of bytes in a row.

   The maximum string or BLOB length can be lowered at run-time using the sqlite3_limit(db,SQLITE_LIMIT_LENGTH,size) interface.

2. Maximum Number Of Columns

   The SQLITE_MAX_COLUMN compile-time parameter is used to set an upper bound on:

   - The number of columns in a table
   - The number of columns in an index
   - The number of columns in a view
   - The number of terms in the SET clause of an UPDATE statement
   - The number of columns in the result set of a SELECT statement
   - The number of terms in a GROUP BY or ORDER BY clause
   - The number of values in an INSERT statement

   The default setting for SQLITE_MAX_COLUMN is 2000. You can change it at compile time to values as large as 32767. On the other hand, many experienced database designers will argue that a well-normalized database will never need more than 100 columns in a table.

   In most applications, the number of columns is small - a few dozen. There are places in the SQLite code generator that use algorithms that are O(N²) where N is the number of columns. So if you redefine SQLITE_MAX_COLUMN to be a really huge number and you generate SQL that uses a large number of columns, you may find that sqlite3_prepare_v2() runs slowly.

   The maximum number of columns can be lowered at run-time using the sqlite3_limit(db,SQLITE_LIMIT_COLUMN,size) interface.

3. Maximum Length Of An SQL Statement

   The maximum number of bytes in the text of an SQL statement is limited to SQLITE_MAX_SQL_LENGTH which defaults to 1000000. You can redefine this limit to be as large as the smaller of SQLITE_MAX_LENGTH and 1073741824.

   If an SQL statement is limited to be a million bytes in length, then obviously you will not be able to insert multi-million byte strings by embedding them as literals inside of INSERT statements. But you should not do that anyway. Use host parameters for your data. Prepare short SQL statements like this:

   > INSERT INTO tab1 VALUES(?,?,?);

   Then use the sqlite3_bind_XXXX() functions to bind your large string values to the SQL statement. The use of binding obviates the need to escape quote characters in the string, reducing the risk of SQL injection attacks. It is also runs faster since the large string does not need to be parsed or copied as much.

   The maximum length of an SQL statement can be lowered at run-time using the sqlite3_limit(db,SQLITE_LIMIT_SQL_LENGTH,size) interface.

4. Maximum Number Of Tables In A Join

   SQLite does not support joins containing more than 64 tables. This limit arises from the fact that the SQLite code generator uses bitmaps with one bit per join-table in the query optimizer.

   SQLite uses an efficient query planner algorithm and so even a large join can be prepared quickly. Hence, there is no mechanism to raise or lower the limit on the number of tables in a join.

5. Maximum Depth Of An Expression Tree

   SQLite parses expressions into a tree for processing. During code generation, SQLite walks this tree recursively. The depth of expression trees is therefore limited in order to avoid using too much stack space.

   The SQLITE_MAX_EXPR_DEPTH parameter determines the maximum expression tree depth. If the value is 0, then no limit is enforced. The current implementation has a default value of 1000.

   The maximum depth of an expression tree can be lowered at run-time using the sqlite3_limit(db,SQLITE_LIMIT_EXPR_DEPTH,size) interface if the SQLITE_MAX_EXPR_DEPTH is initially positive. In other words, the maximum expression depth can be lowered at run-time if there is already a compile-time limit on the expression depth. If SQLITE_MAX_EXPR_DEPTH is set to 0 at compile time (if the depth of expressions is unlimited) then the sqlite3_limit(db,SQLITE_LIMIT_EXPR_DEPTH,size) is a no-op.

6. Maximum Number Of Arguments On A Function

   The SQLITE_MAX_FUNCTION_ARG parameter determines the maximum number of parameters that can be passed to an SQL function. The default value of this limit is 100. SQLite should work with functions that have thousands of parameters. However, we suspect that anybody who tries to invoke a function with more than a few parameters is really trying to find security exploits in systems that use SQLite, not do useful work, and so for that reason we have set this parameter relatively low.

   The number of arguments to a function is sometimes stored in a signed character. So there is a hard upper bound on SQLITE_MAX_FUNCTION_ARG of 127.

   The maximum number of arguments in a function can be lowered at run-time using the sqlite3_limit(db,SQLITE_LIMIT_FUNCTION_ARG,size) interface.

7. Maximum Number Of Terms In A Compound SELECT Statement

   A compound SELECT statement is two or more SELECT statements connected by operators UNION, UNION ALL, EXCEPT, or INTERSECT. We call each individual SELECT statement within a compound SELECT a "term".

   The code generator in SQLite processes compound SELECT statements using a recursive algorithm. In order to limit the size of the stack, we therefore limit the number of terms in a compound SELECT. The maximum number of terms is SQLITE_MAX_COMPOUND_SELECT which defaults to 500. We think this is a generous allotment since in practice we almost never see the number of terms in a compound select exceed single digits.

   The maximum number of compound SELECT terms can be lowered at run-time using the sqlite3_limit(db,SQLITE_LIMIT_COMPOUND_SELECT,size) interface.

8. Maximum Length Of A LIKE Or GLOB Pattern

   The pattern matching algorithm used in the default LIKE and GLOB implementation of SQLite can exhibit O(N²) performance (where N is the number of characters in the pattern) for certain pathological cases. To avoid denial-of-service attacks from miscreants who are able to specify their own LIKE or GLOB patterns, the length of the LIKE or GLOB pattern is limited to SQLITE_MAX_LIKE_PATTERN_LENGTH bytes. The default value of this limit is 50000. A modern workstation can evaluate even a pathological LIKE or GLOB pattern of 50000 bytes relatively quickly. The denial of service problem only comes into play when the pattern length gets into millions of bytes. Nevertheless, since most useful LIKE or GLOB patterns are at most a few dozen bytes in length, paranoid application developers may want to reduce this parameter to something in the range of a few hundred if they know that external users are able to generate arbitrary patterns.

   The maximum length of a LIKE or GLOB pattern can be lowered at run-time using the sqlite3_limit(db,SQLITE_LIMIT_LIKE_PATTERN_LENGTH,size) interface.

9. Maximum Number Of Host Parameters In A Single SQL Statement

   A host parameter is a place-holder in an SQL statement that is filled in using one of the sqlite3_bind_XXXX() interfaces. Many SQL programmers are familiar with using a question mark ("?") as a host parameter. SQLite also supports named host parameters prefaced by ":", "$", or "@" and numbered host parameters of the form "?123".

   Each host parameter in an SQLite statement is assigned a number. The numbers normally begin with 1 and increase by one with each new parameter. However, when the "?123" form is used, the host parameter number is the number that follows the question mark.

   SQLite allocates space to hold all host parameters between 1 and the largest host parameter number used. Hence, an SQL statement that contains a host parameter like ?1000000000 would require gigabytes of storage. This could easily overwhelm the resources of the host machine. To prevent excessive memory allocations, the maximum value of a host parameter number is SQLITE_MAX_VARIABLE_NUMBER, which defaults to 999.

   The maximum host parameter number can be lowered at run-time using the sqlite3_limit(db,SQLITE_LIMIT_VARIABLE_NUMBER,size) interface.

10. Maximum Depth Of Trigger Recursion

    SQLite limits the depth of recursion of triggers in order to prevent a statement involving recursive triggers from using an unbounded amount of memory.

    Prior to SQLite version 3.6.18, triggers were not recursive and so this limit was meaningless. Beginning with version 3.6.18, recursive triggers were supported but had to be explicitly enabled using the PRAGMA recursive_triggers statement. Beginning with version 3.7.0, recursive triggers are enabled by default but can be manually disabled using PRAGMA recursive_triggers. The SQLITE_MAX_TRIGGER_DEPTH is only meaningful if recursive triggers are enabled.

    The default maximum trigger recursion depth is 1000.

11. Maximum Number Of Attached Databases

    The ATTACH statement is an SQLite extension that allows two or more databases to be associated to the same database connection and to operate as if they were a single database. The number of simultaneously attached databases is limited to SQLITE_MAX_ATTACHED which is set to 10 by default. The maximum number of attached databases cannot be increased above 125.

    The maximum number of attached databases can be lowered at run-time using the sqlite3_limit(db,SQLITE_LIMIT_ATTACHED,size) interface.

12. Maximum Number Of Pages In A Database File

    SQLite is able to limit the size of a database file to prevent the database file from growing too large and consuming too much disk space. The SQLITE_MAX_PAGE_COUNT parameter, which is normally set to 1073741823, is the maximum number of pages allowed in a single database file. An attempt to insert new data that would cause the database file to grow larger than this will return SQLITE_FULL.

    The largest possible setting for SQLITE_MAX_PAGE_COUNT is 2147483646. When used with the maximum page size of 65536, this gives a maximum SQLite database size of about 140 terabytes.

    The max_page_count PRAGMA can be used to raise or lower this limit at run-time.

13. Maximum Number Of Rows In A Table

    The theoretical maximum number of rows in a table is 264 (18446744073709551616 or about 1.8e+19). This limit is unreachable since the maximum database size of 140 terabytes will be reached first. A 140 terabytes database can hold no more than approximately 1e+13 rows, and then only if there are no indices and if each row contains very little data.

14. Maximum Database Size

    Every database consists of one or more "pages". Within a single database, every page is the same size, but different database can have page sizes that are powers of two between 512 and 65536, inclusive. The maximum size of a database file is 2147483646 pages. At the maximum page size of 65536 bytes, this translates into a maximum database size of approximately 1.4e+14 bytes (140 terabytes, or 128 tebibytes, or 140,000 gigabytes or 128,000 gibibytes).

    This particular upper bound is untested since the developers do not have access to hardware capable of reaching this limit. However, tests do verify that SQLite behaves correctly and sanely when a database reaches the maximum file size of the underlying filesystem (which is usually much less than the maximum theoretical database size) and when a database is unable to grow due to disk space exhaustion.

15. Maximum Number Of Tables In A Schema

    Each table and index requires at least one page in the database file. An "index" in the previous sentence means an index created explicitly using a CREATE INDEX statement or implicit indices created by UNIQUE and PRIMARY KEY constraints. Since the maximum number of pages in a database file is 2147483646 (a little over 2 billion) this is also then an upper bound on the number of tables and indices in a schema.

    Whenever a database is opened, the entire schema is scanned and parsed and a parse tree for the schema is held in memory. That means that database connection startup time and initial memory usage is proportional to the size of the schema.