Support for generated columns

Added DDL support for "computed columns"; these are DDL column
specifications for columns that have a server-computed value, either upon
SELECT (known as "virtual") or at the point of which they are INSERTed or
UPDATEd (known as "stored").  Support is established for Postgresql, MySQL,
Oracle SQL Server and Firebird. Thanks to Federico Caselli for lots of work
on this one.

ORM round trip tests included.  The ORM makes use of existing
FetchedValue support and no additional ORM logic is present for
the basic feature.

It has been observed that Oracle RETURNING does not return the
new value of a computed column upon UPDATE; it returns the
prior value.  As this is very dangerous, a warning is emitted
if a computed column is rendered into the RETURNING clause
of an UPDATE statement.

Fixes: #4894
Closes: #4928
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4928
Pull-request-sha: d39c521d5ac6ebfb4fb5b53846451de79752e64c

Change-Id: I2610b2999a5b1b127ed927dcdaeee98b769643ce

6 files changed, 74 insertions, 6 deletions

diff --git a/lib/sqlalchemy/dialects/firebird/base.py b/lib/sqlalchemy/dialects/firebird/base.py
index c7c921cb4..052ecf05b 100644
--- a/lib/sqlalchemy/dialects/firebird/base.py
+++ b/lib/sqlalchemy/dialects/firebird/base.py
@@ -581,6 +581,17 @@ class FBDDLCompiler(sql.compiler.DDLCompiler):
                 drop.element
             )
 
+    def visit_computed_column(self, generated):
+        if generated.persisted is not None:
+            raise exc.CompileError(
+                "Firebird computed columns do not support a persistence "
+                "method setting; set the 'persisted' flag to None for "
+                "Firebird support."
+            )
+        return "GENERATED ALWAYS AS (%s)" % self.sql_compiler.process(
+            generated.sqltext, include_table=False, literal_binds=True
+        )
+
 
 class FBIdentifierPreparer(sql.compiler.IdentifierPreparer):
     """Install Firebird specific reserved words."""
diff --git a/lib/sqlalchemy/dialects/mssql/base.py b/lib/sqlalchemy/dialects/mssql/base.py
index 6c7b732ce..5d4de4a33 100644
--- a/lib/sqlalchemy/dialects/mssql/base.py
+++ b/lib/sqlalchemy/dialects/mssql/base.py
@@ -1942,13 +1942,15 @@ class MSSQLStrictCompiler(MSSQLCompiler):
 
 class MSDDLCompiler(compiler.DDLCompiler):
     def get_column_specification(self, column, **kwargs):
-        colspec = (
-            self.preparer.format_column(column)
-            + " "
-            + self.dialect.type_compiler.process(
+        colspec = self.preparer.format_column(column)
+
+        # type is not accepted in a computed column
+        if column.computed is not None:
+            colspec += " " + self.process(column.computed)
+        else:
+            colspec += " " + self.dialect.type_compiler.process(
                 column.type, type_expression=column
             )
-        )
 
         if column.nullable is not None:
             if (
@@ -1958,7 +1960,8 @@ class MSDDLCompiler(compiler.DDLCompiler):
                 or column.autoincrement is True
             ):
                 colspec += " NOT NULL"
-            else:
+            elif column.computed is None:
+                # don't specify "NULL" for computed columns
                 colspec += " NULL"
 
         if column.table is None:
@@ -2110,6 +2113,15 @@ class MSDDLCompiler(compiler.DDLCompiler):
         text += self.define_constraint_deferrability(constraint)
         return text
 
+    def visit_computed_column(self, generated):
+        text = "AS (%s)" % self.sql_compiler.process(
+            generated.sqltext, include_table=False, literal_binds=True
+        )
+        # explicitly check for True|False since None means server default
+        if generated.persisted is True:
+            text += " PERSISTED"
+        return text
+
 
 class MSIdentifierPreparer(compiler.IdentifierPreparer):
     reserved_words = RESERVED_WORDS
diff --git a/lib/sqlalchemy/dialects/mysql/base.py b/lib/sqlalchemy/dialects/mysql/base.py
index 73484aea1..05edb6310 100644
--- a/lib/sqlalchemy/dialects/mysql/base.py
+++ b/lib/sqlalchemy/dialects/mysql/base.py
@@ -1503,6 +1503,9 @@ class MySQLDDLCompiler(compiler.DDLCompiler):
             ),
         ]
 
+        if column.computed is not None:
+            colspec.append(self.process(column.computed))
+
         is_timestamp = isinstance(
             column.type._unwrapped_dialect_impl(self.dialect),
             sqltypes.TIMESTAMP,
diff --git a/lib/sqlalchemy/dialects/oracle/base.py b/lib/sqlalchemy/dialects/oracle/base.py
index 4c5a717b9..c1e91fb12 100644
--- a/lib/sqlalchemy/dialects/oracle/base.py
+++ b/lib/sqlalchemy/dialects/oracle/base.py
@@ -446,6 +446,7 @@ columns for non-unique indexes, all but the last column for unique indexes).
 from itertools import groupby
 import re
 
+from ... import Computed
 from ... import exc
 from ... import schema as sa_schema
 from ... import sql
@@ -905,6 +906,16 @@ class OracleCompiler(compiler.SQLCompiler):
         for i, column in enumerate(
             expression._select_iterables(returning_cols)
         ):
+            if self.isupdate and isinstance(column.server_default, Computed):
+                util.warn(
+                    "Computed columns don't work with Oracle UPDATE "
+                    "statements that use RETURNING; the value of the column "
+                    "*before* the UPDATE takes place is returned.   It is "
+                    "advised to not use RETURNING with an Oracle computed "
+                    "column.  Consider setting implicit_returning to False on "
+                    "the Table object in order to avoid implicit RETURNING "
+                    "clauses from being generated for this Table."
+                )
             if column.type._has_column_expression:
                 col_expr = column.type.column_expression(column)
             else:
@@ -1186,6 +1197,19 @@ class OracleDDLCompiler(compiler.DDLCompiler):
 
         return "".join(table_opts)
 
+    def visit_computed_column(self, generated):
+        text = "GENERATED ALWAYS AS (%s)" % self.sql_compiler.process(
+            generated.sqltext, include_table=False, literal_binds=True
+        )
+        if generated.persisted is True:
+            raise exc.CompileError(
+                "Oracle computed columns do not support 'stored' persistence; "
+                "set the 'persisted' flag to None or False for Oracle support."
+            )
+        elif generated.persisted is False:
+            text += " VIRTUAL"
+        return text
+
 
 class OracleIdentifierPreparer(compiler.IdentifierPreparer):
 
diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index e94f9913c..d6fd2623b 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -1873,6 +1873,9 @@ class PGDDLCompiler(compiler.DDLCompiler):
             if default is not None:
                 colspec += " DEFAULT " + default
 
+        if column.computed is not None:
+            colspec += " " + self.process(column.computed)
+
         if not column.nullable:
             colspec += " NOT NULL"
         return colspec
@@ -2043,6 +2046,18 @@ class PGDDLCompiler(compiler.DDLCompiler):
 
         return "".join(table_opts)
 
+    def visit_computed_column(self, generated):
+        if generated.persisted is False:
+            raise exc.CompileError(
+                "PostrgreSQL computed columns do not support 'virtual' "
+                "persistence; set the 'persisted' flag to None or True for "
+                "PostgreSQL support."
+            )
+
+        return "GENERATED ALWAYS AS (%s) STORED" % self.sql_compiler.process(
+            generated.sqltext, include_table=False, literal_binds=True
+        )
+
 
 class PGTypeCompiler(compiler.GenericTypeCompiler):
     def visit_TSVECTOR(self, type_, **kw):
diff --git a/lib/sqlalchemy/dialects/sqlite/base.py b/lib/sqlalchemy/dialects/sqlite/base.py
index c1f108a0d..02d44a260 100644
--- a/lib/sqlalchemy/dialects/sqlite/base.py
+++ b/lib/sqlalchemy/dialects/sqlite/base.py
@@ -1032,6 +1032,9 @@ class SQLiteCompiler(compiler.SQLCompiler):
 
 class SQLiteDDLCompiler(compiler.DDLCompiler):
     def get_column_specification(self, column, **kwargs):
+        if column.computed is not None:
+            raise exc.CompileError("SQLite does not support computed columns")
+
         coltype = self.dialect.type_compiler.process(
             column.type, type_expression=column
         )