Operationalerror attempt to write a readonly database schema

The general structure can be illustrated as follows:

Operationalerror attempt to write a readonly database schema

We are concerned with things like the maximum number of bytes in a BLOB or the maximum number of columns in a table.

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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 bit integer, then it should work.

Unfortunately, the no-limits policy has been shown to create problems.

operationalerror attempt to write a readonly database schema

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.

operationalerror attempt to write a readonly database schema

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.

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 default value of this macro is 1 billion 1 thousand million or 1,, You can raise or lower this value at compile-time using a command-line option like this: 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.

You can change it at compile time to values as large as On the other hand, many experienced database designers will argue that a well-normalized database will never need more than columns in a table. In most applications, the number of columns is small - a few dozen.

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.

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Use host parameters for your data. Prepare short SQL statements like this: 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. 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. 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. If the value is 0, then no limit is enforced.Currently, whenever a statement changes the schema of the database, all prepared statements will be expired, no matter whether they actually need to be prepared again or not.

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