Oracle 5.0 Reference Manual page 30

• Strict Mode and Standard Error Handling.
standard SQL in a number of ways in which it did not previously. Support for standard SQLSTATE
error messages was also implemented. See
• Data Type.
VARCHAR
65,535 bytes, and stripping of trailing whitespace was eliminated.
efficient than in previous versions, due to the elimination of the old (and nonstandard) removal of
trailing spaces during retrieval. (The actual maximum length of a
maximum row size and the character set you use. The maximum effective column length is subject to
a row size of 65,535 bytes, which is shared among all columns.) See
• Data Type. A true
BIT
notation. This type is much more efficient for storage and retrieval of Boolean values than the
workarounds required in MySQL in versions previous to 5.0. See
Overview".
• Optimizer enhancements.
certain types of queries and in the handling of certain types. These include:
• MySQL 5.0 introduces a new "greedy" optimizer which can greatly reduce the time required to
arrive at a query execution plan. This is particularly noticeable where several tables are to be
joined and no good join keys can otherwise be found. Without the greedy optimizer, the complexity
of the search for an execution plan is calculated as N!, where
joined. The greedy optimizer reduces this to N!/(D-1)!, where
Although the greedy optimizer does not guarantee the best possible of all execution plans (this
is currently being worked on), it can reduce the time spent arriving at an execution plan for a join
involving a great many tables—30, 40, or more—by a factor of as much as 1,000. This should
eliminate most if not all situations where users thought that the optimizer had hung when trying to
perform joins across many tables.
• Use of the Index Merge method to obtain better optimization of
relations over different keys. (Previously, these were optimized only where both relations in the
clause involved the same key.) This also applies to other one-to-one comparison operators
WHERE
(>, <, and so on), including
indexes in retrieving results for conditions such as
combinations of conditions such as
1). See
OR key4 =
• A new equality detector finds and optimizes "hidden" equalities in joins. For example, a
clause such as
t1.c1=t2.c2 AND t2.c2=t3.c3 AND t1.c1 < 5
implies these other conditions
t1.c1=t3.c3 AND t2.c2 < 5 AND t3.c3 < 5
These optimizations can be applied with any combination of
See Section 8.3.1.9, "Nested Join
• Optimization of
NOT IN
queries making use of them by mean of range analysis. The performance of MySQL with regard to
these relations now matches its performance with regard to
• XA Transactions.
Transactions".
What Is New in MySQL 5.0
The effective maximum length of a
column type is available that can be used to store numbers in binary
BIT
Several optimizer improvements were made to improve the speed of
and the operator. This means that MySQL can use multiple
= IN
WHERE (key1 > 4 OR key2 < 7) AND (key3 >= 10
Section 8.3.1.4, "Index Merge
Optimization", and
and
NOT BETWEEN
MySQL 5.0 supports XA (distributed) transactions. See
10
MySQL 5.0 added a strict mode where by it follows
Section 5.1.7, "Server SQL
VARCHAR
VARCHAR
VARCHAR
Section 11.1.1, "Numeric Type
is the number of tables to be
N
D
AND
WHERE key1 > 4 OR key2 < 7
Optimization".
AND
Section 8.3.1.10, "Outer Join
relations, reducing or eliminating table scans for
and BETWEEN.
IN
Modes".
column was increased to
in MySQL 5.0 is more
is determined by the
Section 11.1.6, "String
is the depth of the search.
[880] and [881]
OR
and even
WHERE
[880] and [881] operators.
OR
Simplification".
Section 13.3.7, "XA
Types".