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int sqlite3_vtab_rhs_value(sqlite3_index_info*, int, sqlite3_value **ppVal);
This API may only be used from within the xBestIndex method of a virtual table implementation. The result of calling this interface from outside of an xBestIndex method are undefined and probably harmful.
When the sqlite3_vtab_rhs_value(P,J,V) interface is invoked from within the xBestIndex method of a virtual table implementation, with P being a copy of the sqlite3_index_info object pointer passed into xBestIndex and J being a 0-based index into P->aConstraint[], then this routine attempts to set *V to the value of the right-hand operand of that constraint if the right-hand operand is known. If the right-hand operand is not known, then *V is set to a NULL pointer. The sqlite3_vtab_rhs_value(P,J,V) interface returns SQLITE_OK if and only if *V is set to a value. The sqlite3_vtab_rhs_value(P,J,V) inteface returns SQLITE_NOTFOUND if the right-hand side of the J-th constraint is not available. The sqlite3_vtab_rhs_value() interface can return an result code other than SQLITE_OK or SQLITE_NOTFOUND if something goes wrong.
The sqlite3_vtab_rhs_value() interface is usually only successful if the right-hand operand of a constraint is a literal value in the original SQL statement. If the right-hand operand is an expression or a reference to some other column or a host parameter, then sqlite3_vtab_rhs_value() will probably return SQLITE_NOTFOUND.
Some constraints, such as SQLITE_INDEX_CONSTRAINT_ISNULL and SQLITE_INDEX_CONSTRAINT_ISNOTNULL, have no right-hand operand. For such constraints, sqlite3_vtab_rhs_value() always returns SQLITE_NOTFOUND.
The sqlite3_value object returned in *V is a protected sqlite3_value and remains valid for the duration of the xBestIndex method call. When xBestIndex returns, the sqlite3_value object returned by sqlite3_vtab_rhs_value() is automatically deallocated.
The "_rhs_" in the name of this routine is an abbreviation for "Right-Hand Side".