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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 what 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.

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.

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 32676. You might be able to redefine this value to be as large as billions, though nobody has ever tried doing that so we do not know if it will work. On the other hand, there are people who will argue that a well-normalized database design will never need a value larger than about 100.

   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.

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.

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.

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.

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. We know of no technical reason why SQLite would not work with functions that have millions of parameters. However, we suspect that anybody who tries to invoke a function with millions of 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.

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.

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.

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.

   The maximum value of a host parameter number is SQLITE_MAX_VARIABLE_NUMBER. This setting defaults to 999.

10. 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 simulataneously attached databases is limited to SQLITE_MAX_ATTACHED which is set to 10 by default. The code generator in SQLite uses bitmaps to keep track of attached databases. That means that the number of attached databases cannot be increased above 30 on a 32-bit machine or 62 on a 64-bit machine.

11. Maximum Database Page Size

    An SQLite database file is organized as pages. The size of each page is a power of 2 between 512 and SQLITE_MAX_PAGE_SIZE. The default value for SQLITE_MAX_PAGE_SIZE is 32768. The current implementation will not support a larger value.

    It used to be the case that SQLite would allocate some stack structures whose size was proportional to the maximum page size. For this reason, SQLite would sometimes be compiled with a smaller maximum page size on embedded devices with limited stack memory. But more recent versions of SQLite put these large structures on the heap, not on the stack, so reducing the maximum page size is no longer necessary on embedded devices.

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

    Note that the transaction processing in SQLite requires two bits of heap memory for every page in the database file. For databases of a few megabytes in size, this amounts to only a few hundred bytes of heap memory. But for gigabyte-sized databases the amount of heap memory required is getting into the kilobyte range and for terabyte-sized databases, megabytes of heap memory must be allocated and zeroed at each transaction. SQLite will support very large databases in theory, but the current implementation is optimized for the common SQLite use cases of embedded devices and persistent stores for desktop applications. In other words, SQLite is designed for use with databases sized in kilobytes or megabytes not gigabytes. If you are building an application to work with databases that are hundreds of gigabytes or more in size, then you should perhaps consider using a different database engine that is explicitly designed for such large data sets.

This page last modified 2007/11/14 03:09:36 UTC