Emit v2.0 deprecation warning for "implicit autocommit"

"Implicit autocommit", which is the COMMIT that occurs when a DML or DDL
statement is emitted on a connection, is deprecated and won't be part of
SQLAlchemy 2.0.   A 2.0-style warning is emitted when autocommit takes
effect, so that the calling code may be adjusted to use an explicit
transaction.

As part of this change, DDL methods such as
:meth:`_schema.MetaData.create_all` when used against a
:class:`_engine.Engine` or :class:`_engine.Connection` will run the
operation in a BEGIN block if one is not started already.

The MySQL and MariaDB dialects now query from the information_schema.tables
system view in order to determine if a particular table exists or not.
Previously, the "DESCRIBE" command was used with an exception catch to
detect non-existent,  which would have the undesirable effect of emitting a
ROLLBACK on the connection. There appeared to be legacy encoding issues
which prevented the use of "SHOW TABLES", for this, but as MySQL support is
now at 5.0.2  or above due to :ticket:`4189`, the information_schema tables
are now available in all cases.

Fixes: #4846
Change-Id: I733a7e0e17477a63607fb9931c87c393bbd7ac57

1 files changed, 18 insertions, 31 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/base.py b/lib/sqlalchemy/dialects/mysql/base.py
index 0c9859e79..1003eeca6 100644
--- a/lib/sqlalchemy/dialects/mysql/base.py
+++ b/lib/sqlalchemy/dialects/mysql/base.py
@@ -887,6 +887,7 @@ from collections import defaultdict
 import re
 
 from sqlalchemy import literal_column
+from sqlalchemy import text
 from sqlalchemy.sql import visitors
 from . import reflection as _reflection
 from .enumerated import ENUM
@@ -938,6 +939,7 @@ from ...sql import compiler
 from ...sql import elements
 from ...sql import roles
 from ...sql import util as sql_util
+from ...sql.sqltypes import Unicode
 from ...types import BINARY
 from ...types import BLOB
 from ...types import BOOLEAN
@@ -2708,39 +2710,24 @@ class MySQLDialect(default.DefaultDialect):
         return connection.exec_driver_sql("SELECT DATABASE()").scalar()
 
     def has_table(self, connection, table_name, schema=None):
-        # SHOW TABLE STATUS LIKE and SHOW TABLES LIKE do not function properly
-        # on macosx (and maybe win?) with multibyte table names.
-        #
-        # TODO: if this is not a problem on win, make the strategy swappable
-        # based on platform.  DESCRIBE is slower.
-
-        # [ticket:726]
-        # full_name = self.identifier_preparer.format_table(table,
-        #                                                   use_schema=True)
+        if schema is None:
+            schema = self.default_schema_name
 
-        full_name = ".".join(
-            self.identifier_preparer._quote_free_identifiers(
-                schema, table_name
-            )
+        rs = connection.execute(
+            text(
+                "SELECT * FROM information_schema.tables WHERE "
+                "table_schema = :table_schema AND "
+                "table_name = :table_name"
+            ).bindparams(
+                sql.bindparam("table_schema", type_=Unicode),
+                sql.bindparam("table_name", type_=Unicode),
+            ),
+            {
+                "table_schema": util.text_type(schema),
+                "table_name": util.text_type(table_name),
+            },
         )
-
-        st = "DESCRIBE %s" % full_name
-        rs = None
-        try:
-            try:
-                rs = connection.execution_options(
-                    skip_user_error_events=True
-                ).exec_driver_sql(st)
-                have = rs.fetchone() is not None
-                rs.close()
-                return have
-            except exc.DBAPIError as e:
-                if self._extract_error_code(e.orig) == 1146:
-                    return False
-                raise
-        finally:
-            if rs:
-                rs.close()
+        return bool(rs.scalar())
 
     def has_sequence(self, connection, sequence_name, schema=None):
         if not self.supports_sequences: