Oracle 5.0 Reference Manual page 670

The column information is retrieved from the table using only information in the index tree without
having to do an additional seek to read the actual row. This strategy can be used when the query
uses only columns that are part of a single index.
If the
Extra
of key values. Without
rows but not using it for lookups. For example, if the index is a covering index for the query, the
optimizer may scan it without using it for lookups.
•
Using index for group-by
Similar to the
MySQL found an index that can be used to retrieve all columns of a
without any extra disk access to the actual table. Additionally, the index is used in the most efficient
way so that for each group, only a few index entries are read. For details, see
"GROUP BY
•
Using sort_union(...),
These indicate how index scans are merged for the
Section 8.3.1.4, "Index Merge
•
Using temporary
To resolve the query, MySQL needs to create a temporary table to hold the result. This typically
happens if the query contains
•
Using where
A
WHERE
Unless you specifically intend to fetch or examine all rows from the table, you may have something
wrong in your query if the
[648]. Even if you are using an index for all parts of a
index
if the column can be NULL.
where
•
Using where with pushed condition
This item applies to
Pushdown optimization to improve the efficiency of a direct comparison between a nonindexed
column and a constant. In such cases, the condition is "pushed down" to the cluster's data nodes
and is evaluated on all data nodes simultaneously. This eliminates the need to send nonmatching
rows over the network, and can speed up such queries by a factor of 5 to 10 times over cases where
Condition Pushdown could be but is not used. For more information, see
Condition Pushdown
Output Interpretation
EXPLAIN
You can get a good indication of how good a join is by taking the product of the values in the
column of the
to execute the query. If you restrict queries with the
product also is used to determine which multiple-table
abort. See Section 8.9.2, "Tuning Server
The following example shows how a multiple-table join can be optimized progressively based on the
information provided by EXPLAIN.
Suppose that you have the
EXPLAIN:
EXPLAIN SELECT tt.TicketNumber, tt.TimeIn,
EXPLAIN
column also says
Using
where, the optimizer may be reading the index to avoid reading data
Using
table access method,
Using index
Optimization".
Using
Optimization".
GROUP BY
clause is used to restrict which rows to match against the next table or send to the client.
Extra
NDBCLUSTER
Optimization".
output. This should tell you roughly how many rows MySQL must examine
EXPLAIN
SELECT
Output Format
where, it means the index is being used to perform lookups
Using index for group-by
union(...), Using intersect(...)
index_merge
and
ORDER BY
value is not
Using where
tables only. It means that MySQL Cluster is using the Condition
max_join_size
SELECT
Parameters".
statement shown here and that you plan to examine it using
650
or
GROUP BY
Section 8.3.1.12,
[647] join type. See
clauses that list columns differently.
and the table join type is
clause, you may see
WHERE
Section 8.3.1.5, "Engine
[469] system variable, this row
statements to execute and which to
indicates that
query
DISTINCT
or
ALL
Using
rows