For the FETCH operator, the following input
arguments are shown:
- Fetched table
- The name of the table to be accessed.
- Columns retrieved
- The set of columns to be accessed from the table.
- Sargable predicates
- The set of predicates to be applied as
sargable (that is, that can be resolved by the database manager by simple
comparisons rather than by subqueries).
If the FETCH operator is associated with an index scan (IXSCAN) operator
but is separated from it by other operators, re-apply the predicates of the
index scan as residuals rather than sargables to ensure correct
results.
- Residual predicates
- The set of additional predicates to be applied as residuals.
- Direct fetch
- The row-identifier column that is used to locate the row.
- Prefetch
- A flag that indicates the type of prefetching. Possible values
are:
- Sequential prefetch is enabled.
- A positive number indicating the number of row identifiers (RIDs) for list
prefetch.
- Prefetching is not enabled.
- Maximum pages
- The maximum number of pages that are expected to be read from disk.
Possible values are:
- None - Prefetching is not enabled.
Note: This is different from the prefetch argument, which
indicates whether or not the optimizer expects a prefetch to be done.
Run time is not influenced by the prefetch argument.
- All - All pages of the index or table are expected to be
read. The decision about whether to do a prefetch is made at run
time.
- A positive number - Indicates how many pages are expected to be
read. The closer the number is to zero, the more the prefetch quantity
will be reduced, or prefetch will not be done at all.
- Lock intents
- The lock modes that DB2 uses to access the data for the table, and
optionally to access individual rows. Lock modes include share,
exclusive, update or reuse, or next_key_share. They determine the level
of concurrency that is possible. For the lock mode reuse, the lock mode
for the original base table scan is reused. For more information, see
the section on locking in the Administration Guide.
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