NonStop SQL/MP Reference Manual

NonStop SQL/MP Reference Manual—142115
S-56
General Rules for Similarity
Similarity checks work by comparing information stored in a program file at explicit
compilation time with information current at recompilation time.
During an explicit recompilation, the SQL compiler uses similarity checks to
differentiate between invalid and inoperable plans if you specify COMPILE
INOPERABLE PLANS.
During an automatic recompilation, the SQL executor uses similarity checks to
differentiate between invalid and inoperable plans if you specified CHECK
INOPERABLE PLANS when you last explicitly compiled the program.
General Rules for Similarity
Only tables, protection views, and collations can be similar. A table can be similar to
another table, a protection view can be similar to another protection view, and a
collation can be similar to another collation. No object other than a table, protection
view, or collation is ever considered similar to another object (even an object of the
same type) for the purposes of a similarity check.
A table or protection view compared to a compile-time table or protection view in a
similarity check must have its SIMILARITY CHECK option set to ENABLE.(When
you create a table or view, the default is SIMILARITY CHECK DISABLE, but you can
specify SIMILARITY CHECK ENABLE with the CREATE TABLE, CREATE VIEW,
ALTER TABLE, or ALTER VIEW statements.)
The value of the SIMILARITY CHECK option for a table or protection view affects
only that table or view, not views defined on that table or view.
Similarity Between Protection Views
A protection view referenced at execution time (or recompilation time) is similar to the
protection view used at compilation time if the protection view referenced at execution
time has the SIMILARITY CHECK option enabled and if both protection views
•
Have a similar underlying table
•
Project the same columns from the underlying table
•
Have the same column names
•
Have the same selection expression (determined by a binary comparison of the
parsed, internal representation of the two selection expressions)
Similarity Between Tables
A table referenced at execution time (or recompilation time) is similar to the table used
at compilation time if the table referenced at execution time has the SIMILARITY
CHECK option enabled and if the tables are the same, except in the following aspects:
•
All of the following characteristics can differ:
°
Table names and table statistics
°
Data within the tables