[GitHub] [incubator-superset] villebro commented on a change in pull request #8172: Allow users to estimate query cost before executing it

[GitBox <gi...@apache.org>]

villebro commented on a change in pull request #8172: Allow users to estimate query cost before executing it
URL: https://github.com/apache/incubator-superset/pull/8172#discussion_r324043052
 
 

 ##########
 File path: superset/db_engine_specs/presto.py
 ##########
 @@ -373,6 +380,79 @@ def select_star(
             presto_cols,
         )
 
+    @classmethod
+    def estimate_statement_cost(
+        cls, statement: str, database, cursor, user_name: str
+    ) -> Dict[str, str]:
+        """
+        Generate a SQL query that estimates the cost of a given statement.
+
+        :param statement: A single SQL statement
+        :param database: Database instance
+        :param cursor: Cursor instance
+        :param username: Effective username
+        """
+        db_engine_spec = database.db_engine_spec
+        parsed_query = ParsedQuery(statement)
+        sql = parsed_query.stripped()
+
+        SQL_QUERY_MUTATOR = config.get("SQL_QUERY_MUTATOR")
+        if SQL_QUERY_MUTATOR:
+            sql = SQL_QUERY_MUTATOR(sql, user_name, security_manager, database)
+
+        sql = f"EXPLAIN (TYPE IO, FORMAT JSON) {sql}"
+
+        db_engine_spec.execute(cursor, sql)
+        polled = cursor.poll()
+        while polled:
+            time.sleep(0.2)
+            polled = cursor.poll()
+
+        # the output from Presto is a single column and a single row containing
+        # JSON:
+        #
+        #   {
+        #     ...
+        #     "estimate" : {
+        #       "outputRowCount" : 8.73265878E8,
+        #       "outputSizeInBytes" : 3.41425774958E11,
+        #       "cpuCost" : 3.41425774958E11,
+        #       "maxMemory" : 0.0,
+        #       "networkCost" : 3.41425774958E11
+        #     }
+        #   }
+        data = db_engine_spec.fetch_data(cursor, 1)
+        first = data[0][0]
+        result = json.loads(first)
+        estimate = result["estimate"]
+
+        def humanize(value, suffix):
+            if value == "NaN":
+                return value
+
+            prefixes = ["K", "M", "G", "T", "P", "E", "Z", "Y"]
+            prefix = ""
+            to_next_prefix = 1000
 
 Review comment:
   I had the same concern as @etr2460 , learned something new here (had to google to double check) 👍 

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