Cost in EXPLAIN#
During planning, the cost associated with each node of the plan is computed based on the 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 (XX), cpu: XX, memory: XX, network: XX}
. rows
refers to the expected
number of rows output by each plan node during execution. The value in the
parentheses following the number of rows refers to the expected size of the data
output by each plan node in bytes. Other parameters indicate the estimated
amount of CPU, memory, and network utilized by the execution of a plan node.
These values do not represent any actual unit, but are numbers that are used to
compare the relative costs between plan nodes, allowing the optimizer to choose
the best plan for executing a query. If any of the values is not known, a ?
is printed.
For example:
EXPLAIN SELECT comment FROM tpch.sf1.nation WHERE nationkey > 3;
- Output[comment] => [[comment]]
Estimates: {rows: 22 (1.69kB), cpu: 6148.25, memory: 0.00, network: 1734.25}
- RemoteExchange[GATHER] => [[comment]]
Estimates: {rows: 22 (1.69kB), cpu: 6148.25, memory: 0.00, network: 1734.25}
- ScanFilterProject[table = tpch:nation:sf1.0, filterPredicate = ("nationkey" > BIGINT '3')] => [[comment]]
Estimates: {rows: 25 (1.94kB), cpu: 2207.00, memory: 0.00, network: 0.00}/{rows: 22 (1.69kB), cpu: 4414.00, memory: 0.00, network: 0.00}/{rows: 22 (1.69kB), cpu: 6148.25, memory: 0.00, network: 0.00}
nationkey := tpch:nationkey
comment := tpch:comment
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 are printed, each corresponding to an
individual logical part of the combined operator. For example, three cost
structures are printed for a ScanFilterProject
operator, corresponding
to the Scan
, Filter
, and Project
parts of the operator, in that order.
Estimated cost is also printed in EXPLAIN ANALYZE in addition to actual runtime statistics.