Oracle 5.0 Reference Manual page 668

SELECT * FROM
WHERE key_column
SELECT * FROM
WHERE key_part1= 10 AND
• [648]
index
The
index
ways:
• If the index is a covering index for the queries and can be used to satisify all data required from
the table, only the index tree is scanned. In this case, the
index-only scan usually is faster than
table data.
• A full table scan is performed using reads from the index to look up data rows in index order.
index
MySQL can use this join type when the query uses only columns that are part of a single index.
•
ALL
A full table scan is done for each combination of rows from the previous tables. This is normally not
good if the table is the first table not marked
Normally, you can avoid
constant values or column values from earlier tables.
Extra Information
EXPLAIN
The
Extra
query. The following list explains the values that can appear in this column. If you want to make your
queries as fast as possible, look out for
•
const row not found
For a query such as
•
Distinct
MySQL is looking for distinct values, so it stops searching for more rows for the current row
combination after it has found the first matching row.
•
Full scan on NULL key
This occurs for subquery optimization as a fallback strategy when the optimizer cannot use an index-
lookup access method.
•
Impossible HAVING
The
HAVING
•
Impossible WHERE
The
WHERE
•
Impossible WHERE noticed after reading const tables
MySQL has read all
always false.
•
No matching min/max row
EXPLAIN
tbl_name
IN (10,20,30);
tbl_name
key_part2
join type is the same as ALL, except that the index tree is scanned. This occurs two
does not appear in the
by adding indexes that enable row retrieval from the table based on
ALL
column of output contains additional information about how MySQL resolves the
EXPLAIN
SELECT ... FROM
clause is always false and cannot select any rows.
clause is always false and cannot select any rows.
[646]
const
Output Format
IN (10,20,30);
because the size of the index usually is smaller than the
ALL
column.
Extra
[646], and usually very bad in all other cases.
const
values of
Extra Using filesort
tbl_name, the table was empty.
(and [646]) tables and notice that the
system
648
column says
Extra Using
and
Using
WHERE
index. An
Uses
temporary.
clause is