SQL functions in SQLite can be either "deterministic" or "non-deterministic".
A deterministic function always gives the same answer when it has the same inputs. Most built-in SQL functions in SQLite are deterministic. For example, the abs(X) function always returns the same answer as long as its input X is the same.
Non-deterministic functions might give different answers on each invocation, even if the arguments are always the same. The following are examples of non-deterministic functions:
The random() function is obviously non-deterministic because it gives a different answer every time it is invoked. The answers from changes() and last_insert_rowid() depend on prior SQL statements, and so they are also non-deterministic. The sqlite3_version() function is mostly constant, but it can change when SQLite is upgraded, and so even though it always returns the same answer for any particular session, because it can change answers across sessions it is still considered non-deterministic.
There are some contexts in SQLite that do not allow the use of non-deterministic functions:
In the cases above, the values returned by the function affects the information stored in the database file. The values of functions in CHECK constraints determines which entries are valid for a table, and functions in the WHERE clause of a partial index or in an index on an expression compute values stored in the index b-tree. If any of these functions later returns a different value, then the database might no longer be well-formed. Hence, to avoid database corruption, only deterministic functions can be used in the contexts above.
The built-in date and time functions of SQLite are a special case. These functions are usually considered deterministic. However, if these functions use the string "now" as the date, or if they use the localtime modifier or the utc modifier, then they are considered non-deterministic. Because the function inputs are not necessarily known until run-time, the date/time functions will throw an exception if they encounter any of the non-deterministic features in a context where only deterministic functions are allowed.
Prior to SQLite 3.20.0 (2017-08-01) all date/time functions were always considered non-deterministic. The ability for date/time functions to be deterministic sometimes and non-deterministic at other times, depending on their arguments, was added for the 3.20.0 release.
When the enhancement was made to SQLite 3.20.0 such that date/time functions would be considered deterministic as they do not depend on the current time, one case was overlooked: Many of the date/time functions can be called with no arguments at all. These no-argument date/time functions behave as if they had a single "'now'" argument. Thus "datetime()" and "datetime('now')" both yield the current date and time. However, only the second form was recognized as non-deterministic. This meant that developers could sneak the non-deterministic "datetime()" form into CHECK constraints, index expressions, generated column expressions, and similar places where non-deterministic functions make no sense. This oversight was fixed in version 3.35.2 (2021-03-17). However, there may be legacy databases in circulation that were created by SQLite version 3.20.0 through 3.35.1 that have non-deterministic date/time functions in their schemas.
By default, application-defined SQL functions are considered to be non-deterministic. However, if the 4th parameter to sqlite3_create_function_v2() is OR-ed with SQLITE_DETERMINISTIC, then SQLite will treat that function as if it were deterministic.
Note that if a non-deterministic function is tagged with SQLITE_DETERMINISTIC and if that function ends up being used in the WHERE clause of a partial index or in an expression index, then when the function begins to return different answers, the associated index may become corrupt. If an SQL function is nearly deterministic (which is to say, if it only rarely changes, like sqlite_version()) and it is used in an index that becomes corrupt, the corruption can be fixed by running REINDEX.
The interfaces necessary to construct a function that is sometimes deterministic and sometimes non-deterministic depending on their inputs, such as the built-in date/time functions, are not published. Generic application-defined SQL functions must be always deterministic or always non-deterministic.