SQL/MP Reference Manual

HP NonStop SQL/MP Reference Manual—523352-013
C-69
Constraints
For sequential insert operations, the disk process acquires a range protector lock
on the row that follows the last row inserted. If the last row inserted is at the end of
the file, the range protector lock is placed at the end of the file; consequently, other
servers cannot insert rows at the end of the table or view.
For inserts into a key-sequenced table that uses a SYSKEY column or a timestamp
as the primary key, VSBB is the usual method for insert operations. If concurrent
servers are inserting records into the table, a high percentage of lock waits and
timeouts might occur.
If you are experiencing concurrency problems during insert operations, you should
disable VSBB by specifying this directive:
CONTROL TABLE * SEQUENTIAL INSERT OFF
An application designed for NonStop SQL/MP version 1 might experience
concurrency problems under version 2 or later because NonStop SQL/MP
extended VSBB to update and insert operations in version 2. If a change in
concurrency occurs when you move an application from version 1, check VSBB
usage.
Constraints
Constraints are SQL objects that help to protect the integrity of data in a table by
specifying a condition or conditions that all the values in a particular column of the
table must satisfy.
Adding a constraint allows you to determine whether values exist that violate the
constraint because SQL rejects the constraint if such values exist. Adding the
constraint also keeps such values from being added to the table because SQL rejects
any such values after the constraint is in place.
A constraint name is an SQL identifier.
For more information, see CREATE CONSTRAINT Statement on page C-139.