NonStop SQL/MP Reference Manual

NonStop SQL/MP Reference Manual—142115
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Examples—EXPLAIN
Examples—EXPLAIN
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The following examples show four different ways to request an EXPLAIN report for
the same simple query:
Example 1: >> EXPLAIN SELECT * FROM MYTABLE;
Example 2. >> EXPLAIN "SELECT * FROM MYTABLE";
Example 3. >> EXPLAIN 'SELECT * FROM MYTABLE';
Example 4. >> PREPARE MYQUERY FROM SELECT * FROM MYTABLE;
..
--- SQL command prepared.
>> EXPLAIN MYQUERY;
Total cost
The total cost of executing the statement. Cost is a relative
measure of the resources needed. Lower numbers indicate less
costly execution.
This measure is useful only for comparing different ways of
specifying the same query (for example, using a join instead of
a subquery). Total Cost cannot not be used to compare the
efficiency of executing different queries.
Type of [DELETE]
[UPDATE]
Indicates the type of DELETE or UPDATE operations: cursor,
subset, or unique.
For a cursor DELETE or UPDATE, the executor opens an
internal cursor to read the qualified rows and performs the
requested operation for each such row.
For a subset DELETE or UPDATE, the file system positions on
the first qualifying row (if the row has been determined) or on
the first row of the table. The file system sends a message to the
disk process to perform the operation on all rows that qualify.
The file system might have to send multiple requests because
the disk process performs a limited amount of work in a single
request.
For a unique DELETE or UPDATE, the disk process performs
the operation on the specific row. Exactly one row is deleted or
updated.
UPS workspace
Indicates the size of the workspace needed for the User Process
Sort operation.