Known Limitations Of Mysql Cluster - Oracle 5.0 Reference Manual

In this section, we discuss changes in the implementation of MySQL Cluster in MySQL 5.0 as
compared to MySQL 4.1.
There are relatively few changes between the NDB storage engine implementations in MySQL 4.1 and
in 5.0, so the upgrade path should be relatively quick and painless.
All significantly new features being developed for MySQL Cluster are going into the MySQL Cluster
NDB 7.x trees. For information on changes in the Cluster implementations in MySQL versions 5.1 and
later, see Section 17.1.4, "MySQL Cluster Development
17.1.4.1. MySQL Cluster Developement in MySQL 5.0
MySQL Cluster in MySQL 5.0 contains a number of features added since MySQL 4.1 that are likely to
be of interest:
• Condition pushdown.
SELECT * FROM t1 WHERE non_indexed_attribute = 1;
This query uses a full table scan and the condition is evaluated in the cluster's data nodes. Thus, it
is not necessary to send the records across the network for evaluation. (That is, function transport
is used, rather than data transport.) Please note that this feature is currently disabled by default
(pending more thorough testing), but it should work in most cases. This feature can be enabled
through the use of the
can run
mysqld
condition-pushdown
A major benefit of this change is that queries can be executed in parallel. This means that queries
against nonindexed columns can run faster than previously by a factor of as much as 5 to 10 times,
times the number of data nodes, because multiple CPUs can work on the query in parallel.
You can use
"EXPLAIN
• Decreased
of index memory, and every unique index uses 25 bytes per record of index memory (in addition to
some data memory because these are stored in a separate table). This is because the primary key is
not stored in the index memory anymore.
• Query Cache Enabled for MySQL Cluster: See
information on configuring and using the query cache.
• New optimizations.
interface is now used in some queries. For example, consider the following query:
SELECT * FROM t1 WHERE
This query will be executed 2 to 3 times more quickly than in previous MySQL Cluster versions due
to the fact that all 10 key lookups are sent in a single batch rather than one at a time.
• Limit On Number of Metadata Objects: Beginning with MySQL 5.0.6, each Cluster database
may contain a maximum of 20320 metadata objects—this includes database tables, system tables,
indexes and

17.1.5. Known Limitations of MySQL Cluster

In the sections that follow, we discuss known limitations of MySQL Cluster in MySQL 5.0 as compared
with the features available when using the
no plans to address these in coming releases of MySQL 5.0; however, we will attempt to supply fixes
for these issues in subsequent release series. If you check the "Cluster" category in the MySQL bugs
Known Limitations of MySQL Cluster
Consider the following query:
SET engine_condition_pushdown = On
with the this feature enabled by starting the MySQL server with the
option.
to determine when condition pushdown is being used. See
EXPLAIN
Syntax".
Usage: In MySQL 5.0, each record consumes approximately 25 bytes
IndexMemory
One optimization that merits particular attention is that a batched read
primary_key
values. (Previously, this number was 1600.)
BLOB
History".
Section 8.6.3, "The MySQL Query
IN (1,2,3,4,5,6,7,8,9,10);
and
MyISAM InnoDB
1522
statement. Alternatively, you
--engine-
Section 13.8.2,
Cache", for
storage engines. Currently, there are