Revert naive O(1) table_exists? implementation.

It was a bad idea to rescue exceptions here. This can interfere with
transaction rollbacks which seems to be the cause of current CI
failure.

Instead, each adapter should implement its own DB-specific O(1)
implementation, and we fall back on the generic, slower, implementation
otherwise.

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -16,17 +16,12 @@ module ActiveRecord
         table_name[0...table_alias_length].gsub(/\./, '_')
       end
 
-      # def tables(name = nil) end
-
       # Checks to see if the table +table_name+ exists on the database.
       #
       # === Example
       #   table_exists?(:developers)
       def table_exists?(table_name)
-        select_value("SELECT 1 FROM #{quote_table_name(table_name)} where 1=0", 'SCHEMA')
-        true
-      rescue
-        false
+        tables.include?(table_name.to_s)
       end
 
       # Returns an array of indexes for the given table.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -363,8 +363,10 @@ module ActiveRecord
         show_variable 'collation_database'
       end
 
-      def tables(name = nil, database = nil) #:nodoc:
-        sql = ["SHOW TABLES", database].compact.join(' IN ')
+      def tables(name = nil, database = nil, like = nil) #:nodoc:
+        sql = "SHOW TABLES "
+        sql << "IN #{database} " if database
+        sql << "LIKE #{quote(like)}" if like
 
         execute_and_free(sql, 'SCHEMA') do |result|
           result.collect { |field| field.first }
@@ -372,7 +374,8 @@ module ActiveRecord
       end
 
       def table_exists?(name)
-        return true if super
+        return false unless name
+        return true if tables(nil, nil, name).any?
 
         name          = name.to_s
         schema, table = name.split('.', 2)
@@ -382,7 +385,7 @@ module ActiveRecord
           schema = nil
         end
 
-        tables(nil, schema).include? table
+        tables(nil, schema, table).any?
       end
 
       # Returns an array of indexes for the given table.

activerecord/lib/active_record/connection_adapters/sqlite_adapter.rb

@@ -329,18 +329,23 @@ module ActiveRecord
 
       # SCHEMA STATEMENTS ========================================
 
-      def tables(name = 'SCHEMA') #:nodoc:
+      def tables(name = 'SCHEMA', table_name = nil) #:nodoc:
         sql = <<-SQL
           SELECT name
           FROM sqlite_master
           WHERE type = 'table' AND NOT name = 'sqlite_sequence'
         SQL
+        sql << " AND name = #{quote_table_name(table_name)}" if table_name
 
         exec_query(sql, name).map do |row|
           row['name']
         end
       end
 
+      def table_exists?(name)
+        name && tables('SCHEMA', name).any?
+      end
+
       # Returns an array of +SQLiteColumn+ objects for the table specified by +table_name+.
       def columns(table_name, name = nil) #:nodoc:
         table_structure(table_name).map do |field|

activerecord/test/cases/adapter_test.rb

@@ -17,6 +17,7 @@ module ActiveRecord
     def test_table_exists?
       assert @connection.table_exists?("accounts")
       assert !@connection.table_exists?("nonexistingtable")
+      assert !@connection.table_exists?(nil)
     end
 
     def test_indexes