MDEV-30164 System variable for default collationsbb-10.11-bar-collations

This patch adds a way to override default collations
(or "character set collations") for desired character sets.

The SQL standard says:
> Each collation known in an SQL-environment is applicable to one
> or more character sets, and for each character set, one or more
> collations are applicable to it, one of which is associated with
> it as its character set collation.

In MariaDB, character set collations has been hard-coded so far,
e.g. utf8mb4_general_ci has been a hard-coded character set collation
for utf8mb4.

This patch allows to override (globally per server, or per session)
character set collations, so for example, uca1400_ai_ci can be set as a
character set collation for Unicode character sets
(instead of compiled xxx_general_ci).

The array of overridden character set collations is stored in a new
(session and global) system variable @@character_set_collations and
can be set as a comma separated list of charset=collation pairs, e.g.:

SET @@character_set_collations='utf8mb3=uca1400_ai_ci,utf8mb4=uca1400_ai_ci';

The variable is empty by default, which mean use the hard-coded
character set collations (e.g. utf8mb4_general_ci for utf8mb4).

The variable can also be set globally by passing to the server startup command
line, and/or in my.cnf.

1 files changed, 9 insertions, 5 deletions

diff --git a/sql/sql_lex.h b/sql/sql_lex.h
index 2ddd40568f9..44104c97e84 100644
--- a/sql/sql_lex.h
+++ b/sql/sql_lex.h
@@ -4405,19 +4405,23 @@ public:
   bool add_alter_list(LEX_CSTRING par_name, Virtual_column_info *expr,
                       bool par_exists);
   bool add_alter_list(LEX_CSTRING name, LEX_CSTRING new_name, bool exists);
-  bool add_alter_list_item_convert_to_charset(CHARSET_INFO *cs)
+  bool add_alter_list_item_convert_to_charset(Charset_collation_map_st::Used *used,
+                                              const Charset_collation_map_st &map,
+                                              CHARSET_INFO *cs)
   {
-    if (create_info.add_table_option_convert_charset(cs))
+    if (create_info.add_table_option_convert_charset(used, map, cs))
       return true;
     alter_info.flags|= ALTER_CONVERT_TO;
     return false;
   }
   bool
-  add_alter_list_item_convert_to_charset(CHARSET_INFO *cs,
+  add_alter_list_item_convert_to_charset(Charset_collation_map_st::Used *used,
+                                         const Charset_collation_map_st &map,
+                                         CHARSET_INFO *cs,
                                          const Lex_extended_collation_st &cl)
   {
-    if (create_info.add_table_option_convert_charset(cs) ||
-        create_info.add_table_option_convert_collation(cl))
+    if (create_info.add_table_option_convert_charset(used, map, cs) ||
+        create_info.add_table_option_convert_collation(used, map, cl))
       return true;
     alter_info.flags|= ALTER_CONVERT_TO;
     return false;