Load iidbcapabilities and use to build column definition involving un…

…ique constraints (#58)

* Load iidbcapabilities
* with local scope of iidbcapabilities to per session

lib/sqlalchemy_ingres/base.py

@@ -251,15 +251,28 @@ def get_column_specification(self, column, **kwargs):
         if (
             column.identity is not None
             or column.primary_key is True
+            or column.unique is True
             or not column.nullable
             ):
                 colspec += " NOT NULL"
         else:
-            for x in column.table.constraints:
-                if x.contains_column(column):
-                    colspec += " NOT NULL"
-                    break
-
+            # For columns specified as unique:
+            #  - If table is Ingres, column cannot be nullable
+            #  - If table is X100, column can be nullable
+            # https://docs.actian.com/actianx/12.0/index.html#page/SQLRef/Constraints.htm#ww124554
+            for constraint in column.table.constraints:
+                if constraint.contains_column(column):  # Maybe redundant since checked again in for loop
+                    for constraint_column in constraint:
+                        if constraint_column == column:
+                            if (
+                                constraint_column.nullable is False
+                                or constraint_column.unique is True
+                                or constraint_column.primary_key is True
+                                or isinstance(constraint, sqlalchemy.sql.schema.UniqueConstraint)
+                            ):
+                                if self.dialect.iidbcapabilities['DBMS_TYPE'] == 'INGRES':
+                                    colspec += " NOT NULL"
+                                    break
         return colspec
 
     def visit_create_index(self, create):
@@ -392,12 +405,38 @@ class IngresDialect(default.DefaultDialect):
     requires_name_normalization = True
     sequences_optional    = False
     _isolation_lookup = isolation_lookup
+    iidbcapabilities      = None
     # TODO get_isolation_level()
     # TODO _check_max_identifier_length()
 
     def __init__(self, **kwargs):
         default.DefaultDialect.__init__(self, **kwargs)
 
+    def initialize(self, connection):
+        super().initialize(connection)
+        self._load_iidbcapabilities(connection)
+
+    def _load_iidbcapabilities(self, connection):
+        sqltext = """
+            SELECT
+                cap_capability,
+                cap_value
+            FROM
+                iidbcapabilities"""
+
+        rs = None
+        try:
+            rs = connection.exec_driver_sql(sqltext)
+            self.iidbcapabilities = {}
+            for row in rs.fetchall():
+                coldata = {}
+                self.iidbcapabilities[row[0].rstrip()] = row[1].rstrip()
+
+            rs.close()
+        finally:
+            if rs:
+                rs.close()
+
     def get_isolation_level_values(self, connection):
         return list(self._isolation_lookup)