NonStop SQL/MP Reference Manual

NonStop SQL/MP Reference Manual—142115
C-60
Examples—CONCAT
Examples—CONCAT
•
The following clause concatenates the CITY and STATE values and restricts the
formatted result to 25 single-byte characters:
CONCAT ( CITY STRIP, ", ", STATE) AS A25
Assume that a column of the report contains the following. Note that the entry for
North Carolina is truncated to 25 single-byte characters.
AJO, ARIZONA
NEEDLES, CALIFORNIA
WHEAT RIDGE, COLORADO
SECAUCUS, NEW JERSEY
SWEET HOME, OREGON
WINSTON-SALEM, NORTH CARO
Concurrency
Concurrency is access to the same data by two or more processes at the same time. The
degree of concurrency available (that is, whether a process that requests access to data
that is already being accessed is given access or placed in a wait queue) depends on the
purpose of the access (read or update), on the access mode, and on whether virtual
sequential block buffering (VSBB) is used for the access.
NonStop SQL/MP provides concurrent database access for most operations, controlling
access through the locking mechanism and the mechanism for opening and closing
tables. For DML operations and for some DDL operations, the access options and
locking options you select affect the degree of concurrency. See Access Options
on
page A-1, Locking
on page L-44, and WITH SHARED ACCESS OPTION on page W-4
for more information about these options.
Concurrent access is not possible for all DDL and utility operations, especially those
that change timestamps of SQL objects. To maximize concurrency for operations that
would not otherwise allow it, NonStop SQL/MP performs certain DDL operations in
phases:
•
ALTER INDEX, ALTER TABLE, and CREATE INDEX operations that use WITH
SHARED ACCESS allow concurrent access by DML statements throughout all but
a relatively brief commit phase at the end of the operation.
•
ALTER TABLE, ALTER INDEX, CREATE CONSTRAINT, UPDATE
STATISTICS, and CREATE INDEX operations without WITH SHARED ACCESS
allow concurrent access by DML statements that use SELECT with BROWSE or
SHARED access during an initial scan phase, but lock out DML accesses during a
later update phase.
The following three tables show the limits on concurrency that are imposed by DDL and
utility operations: