diff --git a/frappe/core/doctype/communication/communication.py b/frappe/core/doctype/communication/communication.py
index 77ccefba71..4f9f07d803 100644
--- a/frappe/core/doctype/communication/communication.py
+++ b/frappe/core/doctype/communication/communication.py
@@ -265,8 +265,8 @@ def get_permission_query_conditions_for_communication(user):
 			distinct=True, order_by="idx")
 
 		if not accounts:
-			return """tabCommunication.communication_medium!='Email'"""
+			return """`tabCommunication`.communication_medium!='Email'"""
 
 		email_accounts = [ '"%s"'%account.get("email_account") for account in accounts ]
-		return """tabCommunication.email_account in ({email_accounts})"""\
+		return """`tabCommunication`.email_account in ({email_accounts})"""\
 			.format(email_accounts=','.join(email_accounts))
diff --git a/frappe/database/postgres/schema.py b/frappe/database/postgres/schema.py
index 05b6b19a9a..b5129b60bb 100644
--- a/frappe/database/postgres/schema.py
+++ b/frappe/database/postgres/schema.py
@@ -40,7 +40,20 @@ class PostgresTable(DBTable):
 			query.append("ADD COLUMN `{}` {}".format(col.fieldname, col.get_definition()))
 
 		for col in self.change_type:
-			query.append("ALTER COLUMN `{}` TYPE {}".format(col.fieldname, get_definition(col.fieldtype, precision=col.precision, length=col.length)))
+			using_clause = ""
+			if col.fieldtype in ("Datetime"):
+				# The USING option of SET DATA TYPE can actually specify any expression
+				# involving the old values of the row
+				# read more https://www.postgresql.org/docs/9.1/sql-altertable.html
+				using_clause = "USING {}::timestamp without time zone".format(col.fieldname)
+			elif col.fieldtype in ("Check"):
+				using_clause = "USING {}::smallint".format(col.fieldname)
+
+			query.append("ALTER COLUMN {0} TYPE {1} {2}".format(
+				col.fieldname,
+				get_definition(col.fieldtype, precision=col.precision, length=col.length),
+				using_clause)
+			)
 
 		for col in self.set_default:
 			if col.fieldname=="name":
@@ -93,4 +106,4 @@ class PostgresTable(DBTable):
 						fieldname, self.table_name)))
 					raise e
 				else:
-					raise e
\ No newline at end of file
+					raise e
diff --git a/frappe/desk/doctype/todo/todo.py b/frappe/desk/doctype/todo/todo.py
index 45c3874806..c9fa96c716 100644
--- a/frappe/desk/doctype/todo/todo.py
+++ b/frappe/desk/doctype/todo/todo.py
@@ -94,7 +94,7 @@ def get_permission_query_conditions(user):
 	if "System Manager" in frappe.get_roles(user):
 		return None
 	else:
-		return """(tabToDo.owner = {user} or tabToDo.assigned_by = {user})"""\
+		return """(`tabToDo`.owner = {user} or `tabToDo`.assigned_by = {user})"""\
 			.format(user=frappe.db.escape(user))
 
 def has_permission(doc, user):
@@ -108,4 +108,4 @@ def new_todo(description):
 	frappe.get_doc({
 		'doctype': 'ToDo',
 		'description': description
-	}).insert()
\ No newline at end of file
+	}).insert()
diff --git a/frappe/model/__init__.py b/frappe/model/__init__.py
index 5f804e3c36..3a8868ec2c 100644
--- a/frappe/model/__init__.py
+++ b/frappe/model/__init__.py
@@ -76,27 +76,43 @@ def delete_fields(args_dict, delete=0):
 		args_dict = { dt: [field names] }
 	"""
 	import frappe.utils
-	for dt in list(args_dict):
+	for dt in args_dict:
 		fields = args_dict[dt]
-		if not fields: continue
+		if not fields:
+			continue
 
-		frappe.db.sql("""\
+		frappe.db.sql("""
 			DELETE FROM `tabDocField`
-			WHERE parent=%s AND fieldname IN (%s)
-		""" % ('%s', ", ".join(['"' + f + '"' for f in fields])), dt)
+			WHERE parent='%s' AND fieldname IN (%s)
+		""" % (dt, ", ".join(["'{}'".format(f) for f in fields])))
 
-		# Delete the data / column only if delete is specified
-		if not delete: continue
+		# Delete the data/column only if delete is specified
+		if not delete:
+			continue
 
 		if frappe.db.get_value("DocType", dt, "issingle"):
-			frappe.db.sql("""\
+			frappe.db.sql("""
 				DELETE FROM `tabSingles`
-				WHERE doctype=%s AND field IN (%s)
-			""" % ('%s', ", ".join(['"' + f + '"' for f in fields])), dt)
+				WHERE doctype='%s' AND field IN (%s)
+			""" % (dt, ", ".join(["'{}'".format(f) for f in fields])))
 		else:
-			existing_fields = frappe.db.sql("desc `tab%s`" % dt)
+			existing_fields = frappe.db.multisql({
+					"mariadb": "DESC `tab%s`" % dt,
+					"postgres": """
+						SELECT
+ 							COLUMN_NAME
+						FROM
+ 							information_schema.COLUMNS
+						WHERE
+ 							TABLE_NAME = 'tab%s';
+					""" % dt,
+				})
 			existing_fields = existing_fields and [e[0] for e in existing_fields] or []
+			fields_need_to_delete = set(fields) & set(existing_fields)
+			if not fields_need_to_delete:
+				continue
 			query = "ALTER TABLE `tab%s` " % dt + \
-				", ".join(["DROP COLUMN `%s`" % f for f in fields if f in existing_fields])
-			frappe.db.commit()
+				", ".join(["DROP COLUMN `%s`" % f for f in fields_need_to_delete])
 			frappe.db.sql(query)
+		# commit the results to db
+		frappe.db.commit()
diff --git a/frappe/patches/v11_0/set_default_letter_head_source.py b/frappe/patches/v11_0/set_default_letter_head_source.py
index 069f4e3d2e..a43ea397e4 100644
--- a/frappe/patches/v11_0/set_default_letter_head_source.py
+++ b/frappe/patches/v11_0/set_default_letter_head_source.py
@@ -6,4 +6,4 @@ def execute():
     frappe.reload_doctype('Letter Head')
 
     # source of all existing letter heads must be HTML
-    frappe.db.sql('update `tabLetter Head` set source = "HTML"')
\ No newline at end of file
+    frappe.db.sql("update `tabLetter Head` set source = 'HTML'")
diff --git a/frappe/patches/v12_0/init_desk_settings.py b/frappe/patches/v12_0/init_desk_settings.py
index 782ced8a26..31c6cf9207 100644
--- a/frappe/patches/v12_0/init_desk_settings.py
+++ b/frappe/patches/v12_0/init_desk_settings.py
@@ -8,4 +8,4 @@ from frappe.desk.moduleview import get_onboard_items
 def execute():
 	"""Reset the initial customizations for desk, with modules, indices and links."""
 	frappe.reload_doc("core", "doctype", "user")
-	frappe.db.sql("""update tabUser set home_settings = %s""", (''), debug=True)
+	frappe.db.sql("""update `tabUser` set home_settings = %s""", (''), debug=True)
diff --git a/frappe/patches/v12_0/setup_comments_from_communications.py b/frappe/patches/v12_0/setup_comments_from_communications.py
index 1a7a5aef84..b52304bc05 100644
--- a/frappe/patches/v12_0/setup_comments_from_communications.py
+++ b/frappe/patches/v12_0/setup_comments_from_communications.py
@@ -25,4 +25,4 @@ def execute():
         new_comment.db_insert()
 
     # clean up
-    frappe.db.sql('delete from tabCommunication where communication_type = "Comment"')
\ No newline at end of file
+    frappe.db.sql("delete from `tabCommunication` where communication_type = 'Comment'")