Support Existing Table Column Information (#231)

So far, we don't have any information about the DataFrame's 
columns and their types. This patch supports retrieving column 
information of an existing table in the database. 
Moreover, adds a table check when creating a DataFrame from table, 
so that the user can catch the name error early.

greenplumpython/dataframe.py

@@ -988,7 +988,7 @@ def save_as(
             """,
             has_results=False,
         )
-        return DataFrame.from_table(table_name, self._db, schema=schema)
+        return DataFrame.from_table(table_name, self._db, schema=schema if not temp else "pg_temp")
 
     def create_index(
         self,
@@ -1264,3 +1264,25 @@ def from_files(cls, files: list[str], parser: "NormalFunction", db: Database) ->
         raise NotImplementedError(
             "Please import greenplumpython.experimental.file to load the implementation."
         )
+
+    def describe(self) -> dict[str, str]:
+        """
+        Return a dictionary summarising the column information of the dataframe, conditional on the table existing in the database.
+
+        Returns:
+            Dictionary containing the column names and types.
+
+        """
+        assert self._qualified_table_name is not None, f"Dataframe is not saved in database."
+        columns_query = f"""
+                        SELECT attname AS column_name, atttypid::regtype AS data_type
+                        FROM pg_attribute 
+                        WHERE attrelid = '{self._qualified_table_name}'::regclass and attnum > 0;
+                """
+        assert self._db is not None
+        columns_inf_result = list(self._db._execute(columns_query, has_results=True))  # type: ignore reportUnknownVariableType
+        assert columns_inf_result, f"Table {self._qualified_table_name} does not exists."
+        columns_list: dict[str, str] = {
+            d["column_name"]: d["data_type"] for d in columns_inf_result  # type: ignore reportUnknownVariableType
+        }  # type: ignore reportUnknownVariableType
+        return columns_list

tests/test_dataframe.py

@@ -475,6 +475,22 @@ def test_table_distributed_hash(db: gp.Database):
         assert row["distributedby"] == "DISTRIBUTED BY (id)"
 
 
+def test_table_describe(db: gp.Database):
+    columns = {"a": [1, 2, 3], "b": [1, 2, 3]}
+    t = db.create_dataframe(columns=columns)
+    df = t.save_as("const_table_describe", column_names=["a", "b"], schema="test")
+    result = df.describe()
+    assert len(result) == 2
+    df_s = df[["a", "b"]]
+    with pytest.raises(Exception) as exc_info:
+        df_s.describe()
+    assert "Dataframe is not saved in database" in str(exc_info.value)
+    df_not_exist = db.create_dataframe(table_name="not_exist_table")
+    with pytest.raises(Exception) as exc_info:
+        df_not_exist.describe()
+    assert 'relation "not_exist_table" does not exist' in str(exc_info.value)
+
+
 import pandas as pd