Oracle 5.0, Справочное руководство

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Restrictions on Character Sets
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the server itself acts as a TM. Internal XA support is limited by the capabilities of individual storage
engines. Internal XA is required for handling XA transactions that involve more than one storage
engine. The implementation of internal XA requires that a storage engine support two-phase commit at
the table handler level, and currently this is true only for
InnoDB
.
For
XA START
, the
JOIN
and
RESUME
clauses are not supported.
For
XA END
, the
SUSPEND [FOR MIGRATE]
clause is not supported.
The requirement that the
bqual
part of the
xid
value be different for each XA transaction within
a global transaction is a limitation of the current MySQL XA implementation. It is not part of the XA
specification.
If an XA transaction has reached the
PREPARED
state and the MySQL server is killed (for example,
with
kill -9
on Unix) or shuts down abnormally, the transaction can be continued after the server
restarts. However, if the client reconnects and commits the transaction, the transaction will be absent
from the binary log even though it has been committed. This means the data and the binary log have
gone out of synchrony. An implication is that XA cannot be used safely together with replication.
It is possible that the server will roll back a pending XA transaction, even one that has reached the
PREPARED
state. This happens if a client connection terminates and the server continues to run, or if
clients are connected and the server shuts down gracefully. (In the latter case, the server marks each
connection to be terminated, and then rolls back the
PREPARED
XA transaction associated with it.) It
should be possible to commit or roll back a
PREPARED
XA transaction, but this cannot be done without
changes to the binary logging mechanism.
E.6. Restrictions on Character Sets
• Identifiers are stored in
mysql
database tables (
user
,
db
, and so forth) using
utf8
, but identifiers
can contain only characters in the Basic Multilingual Plane (BMP). Supplementary characters are not
permitted in identifiers.
• The
ucs2
character sets has the following restrictions:
• It cannot be used as a client character set, which means that it does not work for
SET NAMES
or
SET CHARACTER SET
. (See Section 10.1.4, “Connection Character Sets and Collations” .)
• It is currently not possible to use
LOAD DATA INFILE
to load data files that use this character
set.
•
FULLTEXT
indexes cannot be created on a column that this character set. However, you can
perform
IN BOOLEAN MODE
searches on the column without an index.
• The
REGEXP
[899] and
RLIKE
[899] operators work in byte-wise fashion, so they are not multi-byte
safe and may produce unexpected results with multi-byte character sets. In addition, these operators
compare characters by their byte values and accented characters may not compare as equal even if
a given collation treats them as equal.
E.7. Limits in MySQL
This section lists current limits in MySQL 5.0.
E.7.1. Limits of Joins
The maximum number of tables that can be referenced in a single join is 61. This also applies to the
number of tables that can be referenced in the definition of a view.
E.7.2. Limits on Number of Databases and Tables
MySQL has no limit on the number of databases. The underlying file system may have a limit on the
number of directories.