Documentation for the generator increment.

doc/sql.extensions/README.ddl.txt

@@ -392,3 +392,73 @@ ALTER { SEQUENCE | GENERATOR } <sequence name> RESTART [ WITH <value> ]
 Syntax present in 2.5 for reference:
 
 ALTER SEQUENCE <sequence name> RESTART WITH <value>
+
+
+16) Increment for sequences.
+(Claudio Valderrama)
+
+For 3.0, the options specified in (15) include INCREMENT:
+{ CREATE | RECREATE } { SEQUENCE | GENERATOR } <sequence name> [ START WITH <value> ] [ INCREMENT [BY] <increment> ]
+CREATE OR ALTER { SEQUENCE | GENERATOR } <sequence name> { RESTART | START WITH <value> } [ INCREMENT [BY] <increment> ]
+ALTER { SEQUENCE | GENERATOR } <sequence name> RESTART [ WITH <value> ] [ INCREMENT [BY] <increment> ]
+
+This is the increment that's applied to the SQL standard way of working with generators:
+NEXT VALUE FOR <sequence name>
+is equivalent to
+gen_id(<sequence name>, <increment>)
+
+The default increment for user generators is one and for system generators, zero. This makes possible
+to apply NEXT VALUE FOR <sys generator>
+since system generators cannot be changed by user requests now. Example:
+
+create sequence seq increment 10; -- starts at zero and changes by 10
+select next value for seq from rdb$database; -- result is 10
+select gen_id(seq, 1) from rdb$database; -- result is 11, gen_id() is not affected by the increment.
+
+If the database is new and no trigger has been defined, doing
+select next value for rdb$procedures from rdb$database;
+many times will produce zero again and again, because it's a system generator.
+
+The increment cannot be zero for user generators. Example:
+
+SQL> create generator g00 increment 0;
+Statement failed, SQLSTATE = 42000
+unsuccessful metadata update
+-CREATE SEQUENCE G00 failed
+-INCREMENT 0 is an illegal option for sequence G00
+
+The change in the value of INCREMENT is a feature that takes effect for each request that starts
+after the change commits. Procedures that are invoked for the first time after the change in INCREMENT
+will use the new value if they contain NEXT VALUE FOR statements. Procedures that are already
+running are not affected because they're cached. Procedures relying on NEXT VALUE FOR do not need
+to be recompiled to see the new increment, but if they are running already, they are loaded and
+no effect is seen. Of course, gen_id(gen, expression) is not affected by the increment.
+
+
+17) More security for system objects.
+(Claudio Valderrama)
+
+For 3.0, system domains cannot be altered:
+
+SQL> alter domain rdb$linger type boolean;
+Statement failed, SQLSTATE = 42000
+unsuccessful metadata update
+-ALTER DOMAIN RDB$LINGER failed
+-System domain RDB$LINGER cannot be modified
+
+The value of a system generator cannot be changed by user requests:
+
+SQL> select gen_id(rdb$procedures, 1) from rdb$database;
+               GEN_ID
+=====================
+Statement failed, SQLSTATE = 42000
+System generator RDB$PROCEDURES cannot be modified
+
+SQL> set generator rdb$procedures to 0;
+Statement failed, SQLSTATE = 42000
+unsuccessful metadata update
+-SET GENERATOR RDB$PROCEDURES failed
+-System generator RDB$PROCEDURES cannot be modified
+
+Also, the name RDB$GENERATORS does not refer anymore to an invisible system
+generator. The name can be applied to an user generator.