WL#3811: Deprecate GLOBAL @@collation_database, @@character_set_database; assignment of SESSION counterparts
Affects: Server-5.7 — Status: Complete
Session collation_database and character_set-database variables have broken support now and completely unsupported by 5.x-objects, such as view, stored routine, triggers, events. Thus, we have two options: - fix full support for all these objects, which seems to be quite a complex task, which requires thorough design phase; - change the semantics of the variables. The proposal is to make them readonly so that they provide a way to get character set/collation of the current database only. If SESSION collation_database, character_set_database are readonly, there is no reason in having GLOBAL counterparts of them. So, the proposal of this item is: 1) Deprecate GLOBAL collation_database, character_set_datavase variables -- deprecate in 5.7, remove in 5.8; 2) Make SESSION collation_database, character_set_database variables read only -- deprecate in 5.7, remove in 5.8; Affected bugs: BUG#35357 "character_set_database does not effects CREATE DATABASE without character set" BUG#27208 If no current database, character_set_database !=character_set_server BUG#27687 ALTER TABLE ... CONVERT TO uses @@collation_database
Documented behavior ==================== MySQL manual never mentions global variables @@collation_database, @@character_set_database. Speaking of their SESSION counterparts, the following information is provided in the manual: --cut: charset.xml The character set and collation for the default database can be determined from the values of the @@character_set_database and @@collation_database system variables. The server sets these variables whenever the default database changes. If there is no default database, the variables have the same value as the corresponding server-level system variables, @@character_set_server and @@collation_server. --end cut --cut: charset.xml SET CHARACTER SET is similar to SET NAMES but sets @@character_set_connection and @@collation_connection to @@character_set_database and @@collation_database. --end cut --cut: database-administration.xml @@character_set_database The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as character_set_server. @@collation_database The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as collation_server. --end cut --cut: sql-syntax.xml LOAD DATA INFILE documentation: The character set indicated by the @@character_set_database system variable is used to interpret the information in the file. --end cut --cut: stored-procedures.xml (DECLARE var TYPE description) For character data types, if there is a CHARACTER SET clause in the declaration, the specified character set and its default collation are used. If there is no such clause, the database character set and collation that are in effect at the time the routine is created are used. (These are given by the values of the character_set_database and collation_database system variables. --end cut Original intent of the implementor ================================== The original intent was to allow one to reset SESSION-level *_database variable to temporarily "alter" the default character set of the current database. E.g. to allow one to do: SET @@collation_database=latin5; LOAD DATA infile 'foo.txt' ... ; -- foo.txt is interpreted as if it was in latin5 character set Or: SET @@collation_database=latin1; CREATE TABLE t1 (a CHAR 10)); -- latin1 is created in latin1 character set, regardless of the character set of the current (default) database. What we have now and how it got broken. ======================================= 1. GLOBAL counterparts of character_set_database/collation_database are never used. The only scenario when they can have any effect is when one issues: SET GLOBAL @@character_set_database=latin5;
SELECT @@character_set_database; -- session counterpart takes its initial value from the global counterpart if there is no current database selected. kostja@bodhi:~/work/mysql-5.0-runtime/sql> mysql -uroot -A -s mysql> SET GLOBAL collation_database=latin5_turkish_ci; kostja@bodhi:~/work/mysql-5.0-runtime/sql> mysql -uroot -A -s mysql> select @@collation_database; @@collation_database latin5_turkish_ci This can have no effect on any SQL statement or data inserted/modified in the database. They were never considered as of any practical use and apparently were added by mistake. 2. Local variables. a) Before BUG#2326 "Charset of table is determined by charset of db only if "USE db;" was fixed, collation_database would have effect on all created tables (regardless of what database they are created in). After it was fixed, it would have no effect on CREATE TABLE statement (presumably a bug in the fix for the bug, as it contradicts with the intent. But this fact plays out nice with our intent to deprecate setting of these variables). Example (current 5.0): mysql> drop database db1; Query OK, 1 row affected (0.00 sec) mysql> drop database db2; Query OK, 1 row affected (0.00 sec) mysql> create database db1 character set utf8; Query OK, 1 row affected (0.01 sec) mysql> create database db2 character set latin5; Query OK, 1 row affected (0.00 sec) mysql> use db1; Database changed mysql> set @@character_set_database=latin1; Query OK, 0 rows affected (0.00 sec) mysql> create table t1 (a char(10)); Query OK, 0 rows affected (0.02 sec) mysql> create table db2.t1 (a char(10)); Query OK, 0 rows affected (0.02 sec) mysql> show create table db1.t1\G *************************** 1. row *************************** Table: t1 Create Table: CREATE TABLE `t1` ( `a` char(10) default NULL ) ENGINE=MyISAM DEFAULT CHARSET=utf8 1 row in set (0.00 sec) mysql> show create table db2.t1\G *************************** 1. row *************************** Table: t1 Create Table: CREATE TABLE `t1` ( `a` char(10) default NULL ) ENGINE=MyISAM DEFAULT CHARSET=latin5 1 row in set (0.01 sec) Version when the bug was fixed: 5.0.0 b) Before BUG#15126 character_set_database is not replicated (LOAD DATA INFILE need it) was fixed, setting of a local variable would break replication, since it was never replicated. After this bug got fixed, it has also become possible to explicitly specify the file character set in the LOAD DATA syntax. Version: 5.0.36. c) The manual is wrong saying that if there is no current database, the setting of @@character_set_database is taken from global variable @@character_set_server. Example: mysql> set global character_set_server=latin5; Query OK, 0 rows affected (0.00 sec) mysql> create database foo; Query OK, 1 row affected (0.00 sec) mysql> use foo; Database changed mysql> drop database foo; Query OK, 0 rows affected (0.00 sec) mysql> select @@character_set_database; +--------------------------+ | @@character_set_database | +--------------------------+ | latin1 | +--------------------------+ This is reported as BUG#27208 "If no current database, character_set_database !=character_set_server" To sum up, the only effect that setting of a local variable can currently have is on LOAD DATA INFILE statement, when it influences the alleged character set of the file. Setting of these variables has no effect on stored programs (triggers, functions, procedures), base tables or views -- all these objects use the character set and collation of the database they belong to.
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