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, 10 insertions, 4 deletions

diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index 6f3f96d573f..924b6de0977 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -2200,10 +2200,12 @@ bool check_duplicates_in_interval(const char *set_or_name,
   Generates an error to the diagnostics area in case of a failure.
 */
 bool Column_definition::
-       prepare_charset_for_string(const Column_derived_attributes *dattr)
+       prepare_charset_for_string(Charset_collation_map_st::Used *used,
+                                  const Charset_collation_map_st &map,
+                                  const Column_derived_attributes *dattr)
 {
   CHARSET_INFO *tmp= charset_collation_attrs().
-                       resolved_to_character_set(dattr->charset());
+                       resolved_to_character_set(used, map, dattr->charset());
   if (!tmp)
     return true;
   charset= tmp;
@@ -12457,8 +12459,10 @@ bool HA_CREATE_INFO::
   {
     // Make sure we don't do double resolution in direct SQL execution
     DBUG_ASSERT(!default_table_charset || thd->stmt_arena->is_stmt_execute());
+    Character_set_collations_used used(thd);
     if (!(default_table_charset=
-            default_cscl.resolved_to_context(ctx)))
+            default_cscl.resolved_to_context(&used,
+                           thd->variables.character_set_collations, ctx)))
       return true;
   }
 
@@ -12469,8 +12473,10 @@ bool HA_CREATE_INFO::
     // Make sure we don't do double resolution in direct SQL execution
     DBUG_ASSERT(!alter_table_convert_to_charset ||
                 thd->stmt_arena->is_stmt_execute());
+    Character_set_collations_used used(thd);
     if (!(alter_table_convert_to_charset=
-            convert_cscl.resolved_to_context(ctx)))
+            convert_cscl.resolved_to_context(&used,
+                           thd->variables.character_set_collations, ctx)))
       return true;
   }
   return false;