Use show create table.

Conflicts:

	activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

activerecord/CHANGELOG.md

@@ -1,5 +1,10 @@
 ## Rails 3.1.4 (unreleased) ##
 
+*   Perf fix: MySQL primary key lookup was still slow for very large
+    tables. *GH 3678*
+
+    *Kenny J*
+
 *   Perf fix: If a table has no primary key, don't repeatedly ask the database for it.
 
     *Julius de Bruijn*

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -572,12 +572,15 @@ def show_variable(name)
 
       # Returns a table's primary key and belonging sequence.
       def pk_and_sequence_for(table)
-        keys = []
-        result = execute("SHOW INDEX FROM #{quote_table_name(table)} WHERE Key_name = 'PRIMARY'", 'SCHEMA')
-        result.each(:symbolize_keys => true, :as => :hash) do |row|
-          keys << row[:Column_name]
+        result = execute("SHOW CREATE TABLE #{quote_table_name(table)}", 'SCHEMA')
+        create_table = result.first[1]
+
+        if create_table.to_s =~ /PRIMARY KEY\s+\((.+)\)/
+          keys = $1.split(",").map { |key| key.gsub(/`/, "") }
+          keys.length == 1 ? [keys.first, nil] : nil
+        else
+          nil
         end
-        keys.length == 1 ? [keys.first, nil] : nil
       end
 
       # Returns just a table's primary key

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -755,13 +755,16 @@ def show_variable(name)
 
       # Returns a table's primary key and belonging sequence.
       def pk_and_sequence_for(table) #:nodoc:
-        keys = []
-        result = execute("SHOW INDEX FROM #{quote_table_name(table)} WHERE Key_name = 'PRIMARY'", 'SCHEMA')
-        result.each_hash do |h|
-          keys << h["Column_name"]
-        end
+        result = execute("SHOW CREATE TABLE #{quote_table_name(table)}", 'SCHEMA')
+        create_table = result.fetch_hash["Create Table"]
         result.free
-        keys.length == 1 ? [keys.first, nil] : nil
+
+        if create_table.to_s =~ /PRIMARY KEY\s+\((.+)\)/
+          keys = $1.split(",").map { |key| key.gsub(/`/, "") }
+          keys.length == 1 ? [keys.first, nil] : nil
+        else
+          nil
+        end
       end
 
       # Returns just a table's primary key