Neoview SQL Reference Manual (R2.3)

defined by the number of rows specified for period. The value period must be
an integer that is greater than zero (period > 0).
SET ROWCOUNT c
is an optional clause that specifies the number of rows in the table. The value c must be
an integer that is greater than or equal to zero (c > 0).
If the ROWCOUNT clause is not specified, Neoview SQL can, in most cases, determine
the actual table row count.
See “SAMPLE Clause” (page 270).
Considerations for UPDATE STATISTICS
Physical Statistics
Physical statistics (index level, nonempty block count, and EOF) are generated for UPDATE
STATISTICS statements unless you use the CLEAR option.
Using Statistics
Use UPDATE STATISTICS to collect and save statistics on columns. The SQL compiler uses
histogram statistics to determine the selectivity of predicates, indexes, and tables. Because
selectivity directly influences the cost of access plans, regular collection of statistics increases the
likelihood that Neoview SQL chooses efficient access plans.
When a user table is changed, either by changing its data significantly or its definition, reexecute
the UPDATE STATISTICS statement for the table.
Authorization and Locking
To run the UPDATE STATISTICS statement against SQL tables, you must have the authority to
read the user table for which statistics are generated. Because the histogram tables are registered
in the schema (for SQL tables) of table, you must have the authority to read and write to this
schema. Then, when the two histogram tables are created, you become the owner of the tables.
UPDATE STATISTICS momentarily locks the definition of the user table during the operation
but not the user table itself. The UPDATE STATISTICS statement uses READ UNCOMMITTED
for the user table.
Transactions
Do not start a transaction before executing UPDATE STATISTICS because UPDATE STATISTICS
runs under that transaction. The transaction auto abort time could be exceeded during the
processing.
If you do not start a transaction for UPDATE STATISTICS, Neoview SQL runs multiple
transactions, breaking down the long transaction.
Generating and Clearing Statistics for Columns
To generate statistics for particular columns, name each column, or name the first and last columns
of a sequence of columns in the table. For example, suppose that a table has consecutive columns
CITY, STATE, ZIP. This list gives a few examples of possible options you can specify:
Multicolumn GroupSingle-Column Group Within
Parentheses
Single-Column Group
ON (CITY, STATE) or ON
(CITY,STATE,ZIP)
ON (CITY),(STATE),(ZIP)ON CITY, STATE, ZIP
ON (CITY) TO (ZIP)ON CITY TO ZIP
ON (ZIP) TO (CITY)ON ZIP TO CITY
188 SQL Statements