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@ -3344,15 +3344,14 @@ server.
@menu
* Standards:: What standards does MySQL follow?
* ANSI mode:: Running MySQL in ANSI mode
* Extensions to ANSI:: MySQL extensions to ANSI SQL92
* Differences from ANSI:: MySQL differences compared to ANSI SQL92
* ANSI mode:: Running MySQL in ANSI mode
* Commit-rollback:: How to cope without @code{COMMIT}-@code{ROLLBACK}
* Bugs:: Known errors and design deficiencies in MySQL
@end menu
@node Standards, Extensions to ANSI, Compatibility, Compatibility
@node Standards, ANSI mode, Compatibility, Compatibility
@subsection What Standards Does MySQL Follow?
Entry level SQL92. ODBC levels 0-2.
@ -3361,7 +3360,40 @@ We are aiming towards supporting the full ANSI SQL99 standard,
but without concessions to speed and quality of the code.
@node Extensions to ANSI, Differences from ANSI, Standards, Compatibility
@node ANSI mode, Extensions to ANSI, Standards, Compatibility
@subsection Running MySQL in ANSI Mode
@cindex running, ANSI mode
@cindex ANSI mode, running
If you start @code{mysqld} with the @code{--ansi} option, the following
behavior of MySQL changes:
@itemize @bullet
@item
@code{||} is string concatenation instead of @code{OR}.
@item
You can have any number of spaces between a function name and the @samp{(}.
This forces all function names to be treated as reserved words.
@item
@samp{"} will be an identifier quote character (like the MySQL
@samp{`} quote character) and not a string quote character.
@item
@code{REAL} will be a synonym for @code{FLOAT} instead of a synonym of
@code{DOUBLE}.
@item
The default transaction isolation level is @code{SERIALIZABLE}.
@xref{SET TRANSACTION}.
@end itemize
This is the same as using @code{--sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,SERIALIZE,ONLY_FULL_GROUP_BY}.
@node Extensions to ANSI, Differences from ANSI, ANSI mode, Compatibility
@subsection MySQL Extensions to ANSI SQL92
@cindex hints
@ -3608,7 +3640,7 @@ SELECT @@t1:=(@@t2:=1)+@@t3:=4,@@t1,@@t2,@@t3;
@end itemize
@node Differences from ANSI, ANSI mode, Extensions to ANSI, Compatibility
@node Differences from ANSI, Bugs, Extensions to ANSI, Compatibility
@subsection MySQL Differences Compared to ANSI SQL92
We try to make MySQL follow the ANSI SQL standard and the
@ -3616,10 +3648,6 @@ ODBC SQL standard, but in some cases MySQL does some things
differently:
@itemize @bullet
@item
@code{--} is only a comment if followed by a white space. @xref{Missing
comments}.
@item
For @code{VARCHAR} columns, trailing spaces are removed when the value is
stored. @xref{Bugs}.
@ -3639,35 +3667,33 @@ This is because we don't think it's good to have to evaluate a lot of
extra conditions in this case.
@end itemize
The following functionality is missing in the current version of
MySQL. For a prioritized list indicating when new extensions
may be added to MySQL, you should consult
@uref{http://www.mysql.com/documentation/manual.php?section=TODO, the
online MySQL TODO list}. That is the latest version of the TODO
list in this manual. @xref{TODO}.
@menu
* Missing Sub-selects:: Sub-selects
* Missing SELECT INTO TABLE:: @code{SELECT INTO TABLE}
* Missing Transactions:: Transactions
* Missing Triggers:: Triggers
* Missing Foreign Keys:: Foreign Keys
* Broken Foreign KEY:: Why We Did Not Implement Foreign Keys
* Missing Views:: Views
* Missing comments:: @samp{--} as the start of a comment
* ANSI diff Sub-selects:: Sub-@code{SELECT}s
* ANSI diff SELECT INTO TABLE:: @code{SELECT INTO TABLE}
* ANSI diff Transactions:: Transactions and Atomic Operations
* ANSI diff Triggers:: Stored Procedures and Triggers
* ANSI diff Foreign Keys:: Foreign Keys
* ANSI diff Views:: Views
* ANSI diff comments:: @samp{--} as the Start of a Comment
@end menu
For a prioritized list indicating when new extensions will be added to
MySQL, you should consult the online MySQL TODO list at
@uref{http://www.mysql.com/documentation/manual.php?section=TODO}.
That is the latest version of the TODO list in this manual. @xref{TODO}.
@node Missing Sub-selects, Missing SELECT INTO TABLE, Differences from ANSI, Differences from ANSI
@subsubsection Sub-selects
@node ANSI diff Sub-selects, ANSI diff SELECT INTO TABLE, Differences from ANSI, Differences from ANSI
@subsubsection Sub-@code{SELECT}s
@cindex sub-selects
MySQL currently only supports sub selects of the form @code{INSERT
MySQL currently only supports nested queries of the form @code{INSERT
... SELECT ...} and @code{REPLACE ... SELECT ...}. You can however use
the function @code{IN()} in other contexts.
Sub-selects are scheduled for implementation in Version 4.x.
In many cases you can rewrite the query without a sub-select:
Meanwhile, you can often rewrite the query without a sub-select:
@example
SELECT * FROM table1 WHERE id IN (SELECT id FROM table2);
@ -3727,7 +3753,8 @@ MySQL 4.0 supports multi-table deletes that can be used to efficiently
delete rows based on information from one table or even from many tables
at the same time.
@node Missing SELECT INTO TABLE, Missing Transactions, Missing Sub-selects, Differences from ANSI
@node ANSI diff SELECT INTO TABLE, ANSI diff Transactions, ANSI diff Sub-selects, Differences from ANSI
@subsubsection @code{SELECT INTO TABLE}
@findex SELECT INTO TABLE
@ -3743,126 +3770,203 @@ tblTemp1.fldOrder_ID > 100;
@end example
Alternatively, you can use @code{SELECT INTO OUTFILE...} or @code{CREATE
TABLE ... SELECT} to solve your problem.
TABLE ... SELECT}.
@node Missing Transactions, Missing Triggers, Missing SELECT INTO TABLE, Differences from ANSI
@subsubsection Transactions
@node ANSI diff Transactions, ANSI diff Triggers, ANSI diff SELECT INTO TABLE, Differences from ANSI
@subsubsection Transactions and Atomic Operations
@findex COMMIT
@findex ROLLBACK
@cindex transactions, support
@cindex transaction-safe tables
@cindex tables, updating
@cindex updating, tables
@cindex @code{InnoDB} tables
@cindex @code{BDB} tables
As MySQL does nowadays support transactions, the following
discussion is only valid if you are only using the non-transaction-safe
table types. @xref{COMMIT}.
MySQL supports transactions with the @code{InnoDB} and @code{BDB}
@code{Transactional table handlers}. @xref{Table types}.
The question is often asked, by the curious and the critical, ``Why is
MySQL not a transactional database?'' or ``Why does MySQL
not support transactions?''
MySQL has made a conscious decision to support another paradigm
for data integrity, ``atomic operations.'' It is our thinking and
experience that atomic operations offer equal or even better integrity
with much better performance. We, nonetheless, appreciate and understand
the transactional database paradigm and plan, within the next few releases,
to introduce transaction-safe tables on a per table basis. We will be
giving our users the possibility to decide if they need the speed of
atomic operations or if they need to use transactional features in their
applications.
However, the non-transactional table types in MySQL such as
@code{MyISAM} follow another paradigm for data integrity called
``@code{Atomic Operations}.'' Atomic operations often offer equal or
even better integrity with much better performance.
With MySQL supporting both paradigms, the user is able to decide if
they need the speed of atomic operations or if they need to use
transactional features in their applications. This choice can be made
on a per-table basis.
How does one use the features of MySQL to maintain rigorous integrity
and how do these features compare with the transactional paradigm?
First, in the transactional paradigm, if your applications are written
in a way that is dependent on the calling of ``rollback'' instead of
``commit'' in critical situations, then transactions are more
convenient. Moreover, transactions ensure that unfinished updates or
@enumerate
@item
In the transactional paradigm, if your applications are written in a
way that is dependent on the calling of @code{ROLLBACK} instead of
@code{COMMIT} in critical situations, then transactions are more
convenient. Transactions also ensure that unfinished updates or
corrupting activities are not committed to the database; the server is
given the opportunity to do an automatic rollback and your database is
saved.
MySQL, in almost all cases, allows you to solve for potential
problems by including simple checks before updates and by running simple
scripts that check the databases for inconsistencies and automatically
repair or warn if such occurs. Note that just by using the
MySQL log or even adding one extra log, one can normally fix
tables perfectly with no data integrity loss.
MySQL, in almost all cases, allows you to resolve potential problems
by including simple checks before updates and by running simple scripts
that check the databases for inconsistencies and automatically repair
or warn if such an inconsistency occurs. Note that just by using the
MySQL log or even adding one extra log, one can normally fix tables
perfectly with no data integrity loss.
Moreover, fatal transactional updates can be rewritten to be
atomic. In fact,we will go so far as to say that all integrity problems
that transactions solve can be done with @code{LOCK TABLES} or atomic updates,
ensuring that you never will get an automatic abort from the database,
which is a common problem with transactional databases.
@item
More often than not, fatal transactional updates can be rewritten to be
atomic. Generally speaking, all integrity problems that transactions
solve can be done with @code{LOCK TABLES} or atomic updates, ensuring
that you never will get an automatic abort from the database, which is
a common problem with transactional databases.
Not even transactions can prevent all loss if the server goes down. In
such cases even a transactional system can lose data. The difference
between different systems lies in just how small the time-lap is where
they could lose data. No system is 100% secure, only ``secure
enough.'' Even Oracle, reputed to be the safest of transactional
databases, is reported to sometimes lose data in such situations.
@item
Even a transactional system can lose data if the server goes down.
The difference between different systems lies in just how small the
time-lap is where they could lose data. No system is 100% secure, only
``secure enough.'' Even Oracle, reputed to be the safest of
transactional databases, is reported to sometimes lose data in such
situations.
To be safe with MySQL, you only need to have backups and have
the update logging turned on. With this you can recover from any
situation that you could with any transactional database. It is, of
course, always good to have backups, independent of which database you
use.
To be safe with MySQL, whether using transactional tables or not, you
only need to have backups and have the update logging turned on. With
this you can recover from any situation that you could with any
other transactional database. It is, of course, always good to have
backups, independent of which database you use.
@end enumerate
The transactional paradigm has its benefits and its drawbacks. Many
users and application developers depend on the ease with which they can
code around problems where an abort appears to be, or is necessary, and they
may have to do a little more work with MySQL to either think
differently or write more. If you are new to the atomic operations
paradigm, or more familiar or more comfortable with transactions, do not
jump to the conclusion that MySQL has not addressed these
issues. Reliability and integrity are foremost in our minds. Recent
estimates indicate that there are more than 1,000,000 @code{mysqld} servers
currently running, many of which are in production environments. We
hear very, very seldom from our users that they have lost any data, and
in almost all of those cases user error is involved. This is, in our
opinion, the best proof of MySQL's stability and reliability.
users and application developers depend on the ease with which they
can code around problems where an abort appears to be, or is necessary.
However, even if you are new to the atomic operations paradigm, or more
familiar with transactions, do consider the speed benefit that
non-transactional tables can offer, on the order of three to five times
the speed of the fastest and most optimally tuned transactional tables.
Lastly, in situations where integrity is of highest importance,
MySQL's current features allow for transaction-level or better
reliability and integrity. If you lock tables with @code{LOCK TABLES}, all
updates will stall until any integrity checks are made. If you only obtain
a read lock (as opposed to a write lock), then reads and inserts are
still allowed to happen. The new inserted records will not be seen by
any of the clients that have a @code{READ} lock until they release their read
locks. With @code{INSERT DELAYED} you can queue inserts into a local queue,
until the locks are released, without having the client wait for the insert
to complete. @xref{INSERT DELAYED}.
In situations where integrity is of highest importance, MySQL offers
transaction-level or better reliability and integrity even for
non-transactional tables.
If you lock tables with @code{LOCK TABLES}, all updates will stall
until any integrity checks are made. If you only obtain a read lock
(as opposed to a write lock), then reads and inserts are still allowed
to happen. The new inserted records will not be seen by any of the
clients that have a @code{READ} lock until they release their read
locks. With @code{INSERT DELAYED} you can queue inserts into a local
queue, until the locks are released, without having the client wait
for the insert to complete. @xref{INSERT DELAYED}.
``Atomic,'' in the sense that we mean it, is nothing magical. It only means
that you can be sure that while each specific update is running, no other
user can interfere with it, and there will never be an automatic
rollback (which can happen on transaction based systems if you are not
very careful). MySQL also guarantees that there will not be
any dirty reads. You can find some example of how to write atomic updates
in the commit-rollback section. @xref{Commit-rollback}.
``Atomic,'' in the sense that we mean it, is nothing magical. It only
means that you can be sure that while each specific update is running,
no other user can interfere with it, and there will never be an
automatic rollback (which can happen with transactional tables if you
are not very careful). MySQL also guarantees that there will not be
any dirty reads.
We have thought quite a bit about integrity and performance, and we
believe that our atomic operations paradigm allows for both high
reliability and extremely high performance, on the order of three to
five times the speed of the fastest and most optimally tuned of
transactional databases. We didn't leave out transactions because they
are hard to do. The main reason we went with atomic operations as
opposed to transactions is that by doing this we could apply many speed
optimizations that would not otherwise have been possible.
Following are some techniques for working with non-transactional tables:
Many of our users who have speed foremost in their minds are not at all
concerned about transactions. For them transactions are not an
issue. For those of our users who are concerned with or have wondered
about transactions vis-a-vis MySQL, there is a ``MySQL
way'' as we have outlined above. For those where safety is more
important than speed, we recommend them to use the @code{InnoDB},
or @code{BDB} tables for all their critical data. @xref{Table types}.
@itemize @bullet
@item
Loops that need transactions normally can be coded with the help of
@code{LOCK TABLES}, and you don't need cursors when you can update
records on the fly.
One final note: We are currently working on a safe replication schema
that we believe to be better than any commercial replication system we
know of. This system will work most reliably under the atomic
operations, non-transactional, paradigm. Stay tuned.
@item
To avoid using @code{ROLLBACK}, you can use the following strategy:
@enumerate
@item
Use @code{LOCK TABLES ...} to lock all the tables you want to access.
@item
Test conditions.
@item
Update if everything is okay.
@item
Use @code{UNLOCK TABLES} to release your locks.
@end enumerate
This is usually a much faster method than using transactions with
possible @code{ROLLBACK}s, although not always. The only situation
this solution doesn't handle is when someone kills the threads in the
middle of an update. In this case, all locks will be released but some
of the updates may not have been executed.
@item
You can also use functions to update records in a single operation.
You can get a very efficient application by using the following
techniques:
@itemize @bullet
@item Modify fields relative to their current value.
@item Update only those fields that actually have changed.
@end itemize
For example, when we are doing updates to some customer information, we
update only the customer data that has changed and test only that none of
the changed data, or data that depend on the changed data, has changed
compared to the original row. The test for changed data is done with the
@code{WHERE} clause in the @code{UPDATE} statement. If the record wasn't
updated, we give the client a message: "Some of the data you have changed
have been changed by another user". Then we show the old row versus the new
row in a window, so the user can decide which version of the customer record
he should use.
This gives us something that is similar to column locking but is actually
even better, because we only update some of the columns, using values that
are relative to their current values. This means that typical @code{UPDATE}
statements look something like these:
@example
UPDATE tablename SET pay_back=pay_back+'relative change';
UPDATE customer
SET
customer_date='current_date',
address='new address',
phone='new phone',
money_he_owes_us=money_he_owes_us+'new_money'
WHERE
customer_id=id AND address='old address' AND phone='old phone';
@end example
As you can see, this is very efficient and works even if another client
has changed the values in the @code{pay_back} or @code{money_he_owes_us}
columns.
@item
@findex mysql_insert_id()
@findex LAST_INSERT_ID()
In many cases, users have wanted @code{ROLLBACK} and/or @code{LOCK
TABLES} for the purpose of managing unique identifiers for some tables.
This can be handled much more efficiently by using an
@code{AUTO_INCREMENT} column and either the SQL function
@code{LAST_INSERT_ID()} or the C API function @code{mysql_insert_id()}.
@xref{mysql_insert_id, , @code{mysql_insert_id()}}.
@cindex rows, locking
Generally, you can code around row-level locking. Some cases really
need it, but they are very few. For instance, you can use a flag
column in the table and do something like this:
@example
UPDATE tbl_name SET row_flag=1 WHERE id=ID;
@end example
MySQL returns 1 for the number of affected rows if the row was
found and @code{row_flag} wasn't already 1 in the original row.
You can think of it as MySQL changed the above query to:
@example
UPDATE tbl_name SET row_flag=1 WHERE id=ID and row_flag <> 1;
@end example
@end itemize
@node Missing Triggers, Missing Foreign Keys, Missing Transactions, Differences from ANSI
@node ANSI diff Triggers, ANSI diff Foreign Keys, ANSI diff Transactions, Differences from ANSI
@subsubsection Stored Procedures and Triggers
@cindex stored procedures and triggers, defined
@ -3882,14 +3986,13 @@ each time a record is deleted from a transaction table and that automatically
deletes the corresponding customer from a customer table when all his
transactions are deleted.
The planned update language will be able to
handle stored procedures, but without triggers. Triggers usually slow
down everything, even queries for which they are not needed.
To see when MySQL might get stored procedures, see @ref{TODO}.
The planned update language will be able to handle stored procedures, but
without triggers. Triggers usually slow down everything, even queries for
which they are not needed. To see when MySQL will get stored procedures,
see @ref{TODO}.
@node Missing Foreign Keys, Broken Foreign KEY, Missing Triggers, Differences from ANSI
@node ANSI diff Foreign Keys, ANSI diff Views, ANSI diff Triggers, Differences from ANSI
@subsubsection Foreign Keys
@cindex foreign keys
@ -3906,113 +4009,81 @@ SELECT * from table1,table2 where table1.id = table2.id;
@xref{JOIN, , @code{JOIN}}. @xref{example-Foreign keys}.
The @code{FOREIGN KEY} syntax in MySQL exists only for compatibility
with other SQL vendors' @code{CREATE TABLE} commands; it doesn't do
anything. The @code{FOREIGN KEY} syntax without @code{ON DELETE ...} is
mostly used for documentation purposes. Some ODBC applications may use this
to produce automatic @code{WHERE} clauses, but this is usually easy to
override. @code{FOREIGN KEY} is sometimes used as a constraint check, but
this check is unnecessary in practice if rows are inserted into the tables in
the right order. MySQL only supports these clauses because some
applications require them to exist (regardless of whether or not they
work).
In MySQL 3.23.44 and up, @code{InnoDB} tables supports checking of
foreign key constraints. @xref{InnoDB}. For other table types, MySQL
does parse the @code{FOREIGN KEY} syntax in @code{CREATE TABLE}
commands, but without further action being taken.
In MySQL, you can work around the problem of @code{ON DELETE
...} not being implemented by adding the appropriate @code{DELETE} statement to
an application when you delete records from a table that has a foreign key.
In practice this is as quick (in some cases quicker) and much more portable
than using foreign keys.
The @code{FOREIGN KEY} syntax without @code{ON DELETE ...} is mostly
used for documentation purposes. Some ODBC applications may use this
to produce automatic @code{WHERE} clauses, but this is usually easy to
override. @code{FOREIGN KEY} is sometimes used as a constraint check,
but this check is unnecessary in practice if rows are inserted into the
tables in the right order.
In MySQL, you can work around the problem of @code{ON DELETE ...} not
being implemented by adding the appropriate @code{DELETE} statement to
an application when you delete records from a table that has a foreign
key. In practice this is as quick (in some cases quicker) and much more
portable than using foreign keys.
In MySQL 4.0 you can use multi-table delete to delete rows from many
tables with one command. @xref{DELETE}.
In the near future we will extend the @code{FOREIGN KEY} implementation so
that at least the information will be saved in the table specification file
and may be retrieved by @code{mysqldump} and ODBC. At a later stage we will
implement the foreign key constraints for application that can't easily be
coded to avoid them.
In the near future we will extend the @code{FOREIGN KEY} implementation
so that the information will be saved in the table specification file
and may be retrieved by @code{mysqldump} and ODBC. At a later stage we
will implement the foreign key constraints for applications that can't
easily be coded to avoid them.
In MySQL 3.23.44 and up, InnoDB tables supports checking of foreign
key constraints. @xref{InnoDB}.
@menu
* Broken Foreign KEY:: Reasons NOT to use foreign keys constraints
@end menu
@node Broken Foreign KEY, Missing Views, Missing Foreign Keys, Differences from ANSI
@subsubsection Why We Did Not Implement Foreign Keys
@cindex foreign keys, why not implemented
Many database scholars and programmers feel very strongly that
referential integrity should be enforced inside the database server. Indeed,
in many cases, this approach is very helpful. However, in talking with many
database users we have observed that foreign keys are often misused, which
can cause severe problems. Even when used properly, it is not a
magic solution for the referential integrity problem, although it does make
things easier in some cases.
Because of the above observations, we did not assign implementing foreign
keys a high priority. Our user base consisted of mostly of developers who
did not mind enforcing referential integerity inside the application code,
and in fact, preferred to do it that way because it gave them more control.
However, in the last couple of years, our user base has expanded a great deal
and we now have many users who would like to have the enforced referential
integrity support inside MySQL. So we will implement the foreign keys in
the near future, although at this point we cannot provide a definite
delivery date.
Do keep in mind that foreign keys are often misused, which can cause
severe problems. Even when used properly, it is not a magic solution for
the referential integrity problem, although it does make things easier
in some cases.
Some advantages of foreign key enforcement:
@itemize @bullet
@item
Assuming proper design of the relations, foreign key constraints will make it
more difficult for a programmer to introduce an inconsistency into the
database
Assuming proper design of the relations, foreign key constraints will
make it more difficult for a programmer to introduce an inconsistency
into the database.
@item
Using cascading updates and deletes can simplify the client code
Using cascading updates and deletes can simplify the client code.
@item
Properly designed foreign key rules aid in documenting relations between
tables
Properly designed foreign key rules aid in documenting relations
between tables.
@end itemize
Disadvantages:
@itemize @bullet
@item
MySQL does not yet support enforced referential integrity, so if your
application depends on it, you will not be able to use it with MySQL until
we implement this feature.
Mistakes, that are easy to make in designing key relations, can cause
severe problems, for example, circular rules, or the wrong combination
of cascading deletes.
@item
Mistakes, that are easy to make in designing key relations, can cause severe
problems, for example, circular rules, or the wrong combination of cascading
deletes.
A properly written application will make sure internally that it is
not violating referential integrity constraints before proceding with
a query. Thus, additionaly checks on the database level will only slow
down performance for such application.
@item
A properly written application will make sure internally that it is not
violating referential integrity constraints before proceding with a query.
Thus, additionaly checks on the database level will only slow down performance
for such application.
@item
It is not uncommon for a DBA to make such a complex topology of relations that
it becomes very difficult, and in some cases impossible to backup or restore
individual tables.
It is not uncommon for a DBA to make such a complex topology of
relations that it becomes very difficult, and in some cases impossible
to backup or restore individual tables.
@end itemize
@node Missing Views, Missing comments, Broken Foreign KEY, Differences from ANSI
@node ANSI diff Views, ANSI diff comments, ANSI diff Foreign Keys, Differences from ANSI
@subsubsection Views
@cindex views
MySQL doesn't yet support views, but we plan to implement these
to about 4.1.
It is planned to implement views in MySQL around Version 4.1.
Views are mostly useful for letting users access a set of relations as one
table (in read-only mode). Many SQL databases don't allow one to update
@ -4020,16 +4091,16 @@ any rows in a view, but you have to do the updates in the separate tables.
As MySQL is mostly used in applications and on web system where
the application writer has full control on the database usage, most of
our users haven't regarded views to be very important. (At least no one
has been interested enough in this to be prepared to finance the
implementation of views).
our users haven't regarded views to be very important.
(At least no one has been interested enough in this to be prepared to
finance the implementation of views).
One doesn't need views in MySQL to restrict access to columns
as MySQL has a very sophisticated privilege
system. @xref{Privilege system}.
as MySQL has a very sophisticated privilege system.
@xref{Privilege system}.
@node Missing comments, , Missing Views, Differences from ANSI
@node ANSI diff comments, , ANSI diff Views, Differences from ANSI
@subsubsection @samp{--} as the Start of a Comment
@cindex comments, starting
@ -4093,175 +4164,7 @@ shell> replace " #" " --" -- text-file-with-funny-comments.sql
@end example
@node ANSI mode, Commit-rollback, Differences from ANSI, Compatibility
@subsection Running MySQL in ANSI Mode
@cindex running, ANSI mode
@cindex ANSI mode, running
If you start @code{mysqld} with the @code{--ansi} option, the following
behavior of MySQL changes:
@itemize @bullet
@item
@code{||} is string concatenation instead of @code{OR}.
@item
You can have any number of spaces between a function name and the @samp{(}.
This forces all function names to be treated as reserved words.
@item
@samp{"} will be an identifier quote character (like the MySQL
@samp{`} quote character) and not a string quote character.
@item
@code{REAL} will be a synonym for @code{FLOAT} instead of a synonym of
@code{DOUBLE}.
@item
The default transaction isolation level is @code{SERIALIZABLE}.
@xref{SET TRANSACTION}.
@end itemize
This is the same as using @code{--sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,SERIALIZE,ONLY_FULL_GROUP_BY}.
@node Commit-rollback, Bugs, ANSI mode, Compatibility
@subsection How to Cope Without @code{COMMIT}/@code{ROLLBACK}
@findex COMMIT
@findex ROLLBACK
@cindex transaction-safe tables
@cindex tables, updating
@cindex updating, tables
@cindex @code{InnoDB} tables
@cindex @code{BDB} tables
The following mostly applies only for @code{ISAM}, @code{MyISAM}, and
@code{HEAP} tables. If you only use transaction-safe tables (@code{InnoDB},
or @code{BDB} tables) in an an update, you can do
@code{COMMIT} and @code{ROLLBACK} also with MySQL.
@xref{COMMIT}.
The problem with handling @code{COMMIT}-@code{ROLLBACK} efficiently with
the above table types would require a completely different table layout
than MySQL uses today. The table type would also need extra
threads that do automatic cleanups on the tables, and the disk usage
would be much higher. This would make these table types about 2-4 times
slower than they are today.
For the moment, we prefer implementing the SQL server language (something
like stored procedures). With this you would very seldom really need
@code{COMMIT}-@code{ROLLBACK.} This would also give much better performance.
Loops that need transactions normally can be coded with the help of
@code{LOCK TABLES}, and you don't need cursors when you can update records
on the fly.
We at TcX had a greater need for a real fast database than a 100%
general database. Whenever we find a way to implement these features without
any speed loss, we will probably do it. For the moment, there are many more
important things to do. Check the TODO for how we prioritize things at
the moment. (Customers with higher levels of support can alter this, so
things may be reprioritized.)
The current problem is actually @code{ROLLBACK}. Without
@code{ROLLBACK}, you can do any kind of @code{COMMIT} action with
@code{LOCK TABLES}. To support @code{ROLLBACK} with the above table
types, MySQL would have to be changed to store all old records
that were updated and revert everything back to the starting point if
@code{ROLLBACK} was issued. For simple cases, this isn't that hard to do
(the current @code{isamlog} could be used for this purpose), but it
would be much more difficult to implement @code{ROLLBACK} for
@code{ALTER/DROP/CREATE TABLE}.
To avoid using @code{ROLLBACK}, you can use the following strategy:
@enumerate
@item
Use @code{LOCK TABLES ...} to lock all the tables you want to access.
@item
Test conditions.
@item
Update if everything is okay.
@item
Use @code{UNLOCK TABLES} to release your locks.
@end enumerate
This is usually a much faster method than using transactions with possible
@code{ROLLBACK}s, although not always. The only situation this solution
doesn't handle is when someone kills the threads in the middle of an
update. In this case, all locks will be released but some of the updates may
not have been executed.
You can also use functions to update records in a single operation.
You can get a very efficient application by using the following techniques:
@itemize @bullet
@item Modify fields relative to their current value.
@item Update only those fields that actually have changed.
@end itemize
For example, when we are doing updates to some customer information, we
update only the customer data that has changed and test only that none of
the changed data, or data that depend on the changed data, has changed
compared to the original row. The test for changed data is done with the
@code{WHERE} clause in the @code{UPDATE} statement. If the record wasn't
updated, we give the client a message: "Some of the data you have changed
have been changed by another user". Then we show the old row versus the new
row in a window, so the user can decide which version of the customer record
he should use.
This gives us something that is similar to column locking but is actually
even better, because we only update some of the columns, using values that
are relative to their current values. This means that typical @code{UPDATE}
statements look something like these:
@example
UPDATE tablename SET pay_back=pay_back+'relative change';
UPDATE customer
SET
customer_date='current_date',
address='new address',
phone='new phone',
money_he_owes_us=money_he_owes_us+'new_money'
WHERE
customer_id=id AND address='old address' AND phone='old phone';
@end example
As you can see, this is very efficient and works even if another client has
changed the values in the @code{pay_back} or @code{money_he_owes_us} columns.
@findex mysql_insert_id()
@findex LAST_INSERT_ID()
In many cases, users have wanted @code{ROLLBACK} and/or @code{LOCK
TABLES} for the purpose of managing unique identifiers for some tables. This
can be handled much more efficiently by using an @code{AUTO_INCREMENT} column
and either the SQL function @code{LAST_INSERT_ID()} or the C API function
@code{mysql_insert_id()}. @xref{mysql_insert_id, , @code{mysql_insert_id()}}.
@cindex rows, locking
At MySQL AB, we have never had any need for row-level locking
because we have always been able to code around it. Some cases really need
row locking, but they are very few. If you want row-level locking, you
can use a flag column in the table and do something like this:
@example
UPDATE tbl_name SET row_flag=1 WHERE id=ID;
@end example
MySQL returns 1 for the number of affected rows if the row was
found and @code{row_flag} wasn't already 1 in the original row.
You can think of it as MySQL changed the above query to:
@example
UPDATE tbl_name SET row_flag=1 WHERE id=ID and row_flag <> 1;
@end example
@node Bugs, , Commit-rollback, Compatibility
@node Bugs, , Differences from ANSI, Compatibility
@subsection Known errors and design deficiencies in MySQL
@cindex bugs, known
@ -28988,7 +28891,7 @@ and when you put commands in a file and tell @code{mysql} to read its
input from that file with @code{mysql < some-file}.
MySQL doesn't support the @samp{--} ANSI SQL comment style.
@xref{Missing comments}.
@xref{ANSI diff comments}.
@node Reserved words, , Comments, Language Structure