void *sqlite3_get_auxdata(sqlite3_context*, int N); void sqlite3_set_auxdata(sqlite3_context*, int N, void*, void (*)(void*));
These functions may be used by (non-aggregate) SQL functions to associate metadata with argument values. If the same value is passed to multiple invocations of the same SQL function during query execution, under some circumstances the associated metadata may be preserved. An example of where this might be useful is in a regular-expression matching function. The compiled version of the regular expression can be stored as metadata associated with the pattern string. Then as long as the pattern string remains the same, the compiled regular expression can be reused on multiple invocations of the same function.
The sqlite3_get_auxdata() interface returns a pointer to the metadata associated by the sqlite3_set_auxdata() function with the Nth argument value to the application-defined function. If there is no metadata associated with the function argument, this sqlite3_get_auxdata() interface returns a NULL pointer.
The sqlite3_set_auxdata(C,N,P,X) interface saves P as metadata for the N-th argument of the application-defined function. Subsequent calls to sqlite3_get_auxdata(C,N) return P from the most recent sqlite3_set_auxdata(C,N,P,X) call if the metadata is still valid or NULL if the metadata has been discarded. After each call to sqlite3_set_auxdata(C,N,P,X) where X is not NULL, SQLite will invoke the destructor function X with parameter P exactly once, when the metadata is discarded. SQLite is free to discard the metadata at any time, including:
Note the last bullet in particular. The destructor X in sqlite3_set_auxdata(C,N,P,X) might be called immediately, before the sqlite3_set_auxdata() interface even returns. Hence sqlite3_set_auxdata() should be called near the end of the function implementation and the function implementation should not make any use of P after sqlite3_set_auxdata() has been called.
In practice, metadata is preserved between function calls for function parameters that are compile-time constants, including literal values and parameters and expressions composed from the same.
These routines must be called from the same thread in which the SQL function is running.