Support enum types (#387)

### What

This PR improves introspection support for enum types.

Comparison operators and aggregation functions on enum types are
implicitly defined in PostgreSQL and thus were not detected prior to
this PR.

### How

We simply collect which enum types are defined and make up the
conventional comparison operators and the min/max aggregation functions
for all enum types.

We test that we can select and aggregate enum types, and that we can
insert enum values which PostgreSQL validates.

We also collect the enum value labels, which we will be using once
ndc-spec v0.1.1 is released.

Author: plcplc

crates/configuration/src/version3/version3.sql

@@ -296,6 +296,41 @@ WITH
     WHERE
       t.typtype = 'd'
   ),
+
+  -- Enum types are scalar types that consist of a finite, enumerated set of
+  -- labelled values. See
+  -- https://www.postgresql.org/docs/current/datatype-enum.html
+  --
+  -- The catalog table `pg_catalog.pg_enum` records the enum types defined in
+  -- the database. See https://www.postgresql.org/docs/current/catalog-pg-enum.html
+  --
+  -- Enum types support certain comparisons and aggregations, but these are not
+  -- registered in any of the catalog tables. Therefore we need some amount of
+  -- special case handling for enum types.
+  --
+  -- Furthermore we are interested in collecting the labels for each enum type
+  -- to reflect in the NDC schema.
+  enum_types AS
+  (
+    SELECT
+      t.oid AS type_id,
+      t.typnamespace AS schema_id,
+      t.typname AS type_name,
+      array_agg(e.enumlabel ORDER BY e.enumsortorder) AS enum_labels
+    FROM
+      pg_catalog.pg_type AS t
+    INNER JOIN
+      -- Until the schema is made part of our model of types we only consider
+      -- those defined in the public schema.
+      unqualified_schemas_for_types_and_procedures as q
+      ON (t.typnamespace = q.schema_id)
+    INNER JOIN
+      pg_enum
+      AS e
+      ON (e.enumtypid = t.oid)
+    GROUP BY (t.oid, t.typnamespace, t.typname)
+  ),
+
   array_types AS
   (
     SELECT
@@ -394,11 +429,30 @@ WITH
       domain_types
   ),
 
+  -- Enum types support the aggregates 'min' and 'max'. However, these are not
+  -- registered as such in `pg_proc`, and so we have to make them them up
+  -- ourselves.
+  enum_aggregates AS
+  (
+    SELECT
+      proc.proname AS proc_name,
+      e.schema_id AS schema_id,
+      e.type_name AS argument_type,
+      e.type_name AS return_type
+    FROM
+      (VALUES
+        ('min'),
+        ('max')
+      )
+      AS proc(proname),
+      enum_types e
+  ),
+
   -- Aggregate functions are recorded across 'pg_proc' and 'pg_aggregate', see
   -- https://www.postgresql.org/docs/current/catalog-pg-proc.html and
   -- https://www.postgresql.org/docs/current/catalog-pg-aggregate.html for
   -- their schema.
-  aggregates AS
+  declared_aggregates AS
   (
     SELECT
       proc.oid AS proc_id,
@@ -440,6 +494,22 @@ WITH
       AND aggregate.aggnumdirectargs = 0
 
   ),
+  aggregates AS
+  (
+    SELECT
+      proc_name,
+      return_type,
+      argument_type
+    FROM
+      declared_aggregates
+    UNION
+    SELECT
+      proc_name,
+      return_type,
+      argument_type
+    FROM enum_aggregates
+  ),
+
   aggregates_cast_extended AS
   (
     WITH
@@ -602,6 +672,30 @@ WITH
     ORDER BY op.oprname
   ),
 
+  -- Enum types are totally ordered and support the conventional comparison operators.
+  -- They are defined implicitly (i.e., not registered in `pg_proc` or
+  -- `pg_operator`) so we have to make up some definitions for them.
+  enum_comparison_operators AS
+  (
+    SELECT
+      op.oprname AS operator_name,
+      e.type_name AS argument1_type,
+      e.type_name AS argument2_type,
+      true AS is_infix
+    FROM
+      (VALUES
+        ('='),
+        ('!='),
+        ('<>'),
+        ('<='),
+        ('>'),
+        ('>='),
+        ('<')
+      )
+      AS op(oprname),
+      enum_types e
+  ),
+
   -- Here, we reunite our binary infix procedures and our binary prefix
   -- procedures under the umbrella of 'comparison_operators'. We do this
   -- here so that we can treat them uniformly form this point on.
@@ -612,6 +706,8 @@ WITH
     SELECT * FROM comparison_infix_operators
     UNION
     SELECT * FROM comparison_procedures
+    UNION
+    SELECT * FROM enum_comparison_operators
   ),
 
   -- Some comparison operators are not defined explicitly for every type they would be

crates/tests/databases-tests/src/citus/snapshots/databases_tests__citus__schema_tests__schema_test__get_schema.snap

@@ -198,6 +198,65 @@ expression: result
         }
       }
     },
+    "card_suit": {
+      "aggregate_functions": {
+        "max": {
+          "result_type": {
+            "type": "named",
+            "name": "card_suit"
+          }
+        },
+        "min": {
+          "result_type": {
+            "type": "named",
+            "name": "card_suit"
+          }
+        }
+      },
+      "comparison_operators": {
+        "_eq": {
+          "type": "equal"
+        },
+        "_gt": {
+          "type": "custom",
+          "argument_type": {
+            "type": "named",
+            "name": "card_suit"
+          }
+        },
+        "_gte": {
+          "type": "custom",
+          "argument_type": {
+            "type": "named",
+            "name": "card_suit"
+          }
+        },
+        "_in": {
+          "type": "in"
+        },
+        "_lt": {
+          "type": "custom",
+          "argument_type": {
+            "type": "named",
+            "name": "card_suit"
+          }
+        },
+        "_lte": {
+          "type": "custom",
+          "argument_type": {
+            "type": "named",
+            "name": "card_suit"
+          }
+        },
+        "_neq": {
+          "type": "custom",
+          "argument_type": {
+            "type": "named",
+            "name": "card_suit"
+          }
+        }
+      }
+    },
     "char": {
       "aggregate_functions": {
         "max": {
@@ -2539,6 +2598,22 @@ expression: result
         }
       }
     },
+    "deck_of_cards": {
+      "fields": {
+        "pips": {
+          "type": {
+            "type": "named",
+            "name": "int2"
+          }
+        },
+        "suit": {
+          "type": {
+            "type": "named",
+            "name": "card_suit"
+          }
+        }
+      }
+    },
     "delete_playlist_track": {
       "fields": {
         "PlaylistId": {
@@ -3711,6 +3786,44 @@ expression: result
         }
       }
     },
+    "v1_insert_deck_of_cards_object": {
+      "fields": {
+        "pips": {
+          "type": {
+            "type": "named",
+            "name": "int2"
+          }
+        },
+        "suit": {
+          "type": {
+            "type": "named",
+            "name": "card_suit"
+          }
+        }
+      }
+    },
+    "v1_insert_deck_of_cards_response": {
+      "description": "Responses from the 'v1_insert_deck_of_cards' procedure",
+      "fields": {
+        "affected_rows": {
+          "description": "The number of rows affected by the mutation",
+          "type": {
+            "type": "named",
+            "name": "int4"
+          }
+        },
+        "returning": {
+          "description": "Data from rows affected by the mutation",
+          "type": {
+            "type": "array",
+            "element_type": {
+              "type": "named",
+              "name": "deck_of_cards"
+            }
+          }
+        }
+      }
+    },
     "v1_insert_even_numbers_object": {
       "fields": {
         "the_number": {
@@ -4113,6 +4226,13 @@ expression: result
         }
       }
     },
+    {
+      "name": "deck_of_cards",
+      "arguments": {},
+      "type": "deck_of_cards",
+      "uniqueness_constraints": {},
+      "foreign_keys": {}
+    },
     {
       "name": "even_numbers",
       "arguments": {},
@@ -4878,6 +4998,22 @@ expression: result
         "name": "v1_insert_Track_response"
       }
     },
+    {
+      "name": "v1_insert_deck_of_cards",
+      "description": "Insert into the deck_of_cards table",
+      "arguments": {
+        "_object": {
+          "type": {
+            "type": "named",
+            "name": "v1_insert_deck_of_cards_object"
+          }
+        }
+      },
+      "result_type": {
+        "type": "named",
+        "name": "v1_insert_deck_of_cards_response"
+      }
+    },
     {
       "name": "v1_insert_even_numbers",
       "description": "Insert into the even_numbers table",