If the sqlite3_vtab_nochange(X) routine is called within the xColumn method of a virtual table, then it might return true if the column is being fetched as part of an UPDATE operation during which the column value will not change. The virtual table implementation can use this hint as permission to substitute a return value that is less expensive to compute and that the corresponding xUpdate method understands as a "no-change" value.
If the xColumn method calls sqlite3_vtab_nochange() and finds that the column is not changed by the UPDATE statement, then the xColumn method can optionally return without setting a result, without calling any of the sqlite3_result_xxxxx() interfaces. In that case, sqlite3_value_nochange(X) will return true for the same column in the xUpdate method.
The sqlite3_vtab_nochange() routine is an optimization. Virtual table implementations should continue to give a correct answer even if the sqlite3_vtab_nochange() interface were to always return false. In the current implementation, the sqlite3_vtab_nochange() interface does always returns false for the enhanced UPDATE FROM statement.
See also lists of Objects, Constants, and Functions.