19.3. Cost in EXPLAIN

During planning, the cost associated with each node of the plan is computed based on the root table statistics for the tables in the query. This calculated cost is printed as part of the output of an EXPLAIN statement.

Cost information is displayed in the plan tree using the format {rows: XX, bytes: XX}. rows refers to the expected number of rows output by each plan node during execution. bytes refers to the expected size of the data output by each plan node in bytes. If any of the values is not known, a ? is printed.

For example:

presto:default> EXPLAIN SELECT comment FROM nation_with_column_stats WHERE nationkey > 3

- Output[comment] => [comment:varchar(152)] {rows: ?, bytes: ?}
- RemoteExchange[GATHER] => comment:varchar(152) {rows: 12, bytes: ?}
    - ScanFilterProject[table = hive:hive:default:nation_with_column_stats,
                                originalConstraint = (""nationkey"" > BIGINT '3'),
                                filterPredicate = (""nationkey"" > BIGINT '3')] => [comment:varchar(152)] {rows: 25, bytes: ?}/{rows: 12, bytes: ?}/{rows: 12, bytes: ?}
            LAYOUT: hive
            nationkey := HiveColumnHandle{clientId=hive, name=nationkey, hiveType=bigint, hiveColumnIndex=0, columnType=REGULAR}
            comment := HiveColumnHandle{clientId=hive, name=comment, hiveType=varchar(152), hiveColumnIndex=3, columnType=REGULAR}

Generally there is only one cost printed for each plan node. However, when a Scan operator is combined with a Filter and/or Project operator, then multiple cost structures will be printed, each corresponding to an individual logical part of the combined meta-operator. For example, for a ScanFilterProject operator three cost structures will be printed.

  • the first will correspond to Scan part of operator
  • the second will correspond to Filter part of opertor
  • the third will corresponde to Project part of operator

Cost is also printed in EXPLAIN ANALYZE output.