Question

create unique index

In DB2 UDB I can create an index using the following syntax

create unique index I_0004 on TABLENAME (a) INCLUDE (b, c, d);

where a, b, c and d are field of the table TABLENAME.

In DB2 for os390 this syntax (the INCLUDE keyword) is not allowed, so I am creating the indexes as follows

create unique index I_0004 on TABLENAME (a);  
create index I_0005 on TABLENAME (a, b, c, d);

Are the two statements above equivalent to the solution with the INCLUDE keyword?

index columns order

And, if I slightly modify the first statement

create index I_0005 on TABLENAME (a, b, c, d) ALLOW REVERSE SCANS;  

is this ALLOW REVERSE SCANS equivalent to creating indexes

create index I_0005 on TABLENAME (a, b, c, d);  
create index I_0005 on TABLENAME (d, c, b, a);  

or does it consider also any combination of the given columns (I mean, a,b,c,d; b,c,d,a; c,d,a,b; and so on…)?

Answer 1

Regarding the UNIQUE INDEX: roughly, yes, a unique index on (a) including (b, c, d) is equivalent to a unique index just on (a) plus a non-unique one on (a, b, c, d) — except of course that, internally, the database engine may be able to use less space &c.

Regarding ALLOW REVERSE SCANS: no, an index on (a, b) that can be reverse-scanned is not equivalent to one that can’t plus one on (b, a) — rather, an index that can be reverse scanned is equivalent to one that can’t plus another on the same columns where each ASC becomes a DESC and vice versa (and ASC is the default when you don’t specify).

Note that since DB2 9.1 reverse scans are allowed by default, see http://publib.boulder.ibm.com/infocenter/db2luw/v9/index.jsp?topic=/com.ibm.db2.udb.rn.doc/doc/c0023548.htm (and, I believe DB2 V8 is now out of support, see http://www-01.ibm.com/support/docview.wss?rs=71&uid=swg21370360 — I think V9.5 is the current version).