EXPLAIN

EXPLAIN is used to provide the execution plan of a statement.

Syntax

  1. EXPLAIN [ANALYZE] SELECT ...

The ANALYZE clause will execute the statement and measure time spent at each plan node and the total rows of the output etc.

Examples

Explains the following query:

  1. EXPLAIN SELECT * FROM monitor where host='host1';
  1. | plan_type     | plan                                                                                                                                                                                                                                                         
  2. | logical_plan  | Projection: monitor.host, monitor.ts, monitor.cpu, monitor.memory
  3.   Filter: monitor.host = Utf8("host1")
  4.     TableScan: monitor projection=[host, ts, cpu, memory], partial_filters=[monitor.host = Utf8("host1")]                                           |
  5. | physical_plan | ProjectionExec: expr=[host@0 as host, ts@1 as ts, cpu@2 as cpu, memory@3 as memory]
  6.   CoalesceBatchesExec: target_batch_size=4096
  7.     FilterExec: host@0 = host1
  8.       RepartitionExec: partitioning=RoundRobinBatch(10)
  9.         ExecutionPlan(PlaceHolder)

The column plan_type indicates whether it’s alogical_plan or physical_plan. And the column plan explains the plan in detail.

Explains the execution of the plan by ANALYZE:

  1. EXPLAIN ANALYZE SELECT * FROM monitor where host='host1';
  1. | plan_type         | plan
  2. | Plan with Metrics | CoalescePartitionsExec, metrics=[output_rows=1, elapsed_compute=79.167µs, spill_count=0, spilled_bytes=0, mem_used=0]
  3.   ProjectionExec: expr=[host@0 as host, ts@1 as ts, cpu@2 as cpu, memory@3 as memory], metrics=[output_rows=1, elapsed_compute=17.176µs, spill_count=0, spilled_bytes=0, mem_used=0]
  4.     CoalesceBatchesExec: target_batch_size=4096, metrics=[output_rows=1, elapsed_compute=14.248µs, spill_count=0, spilled_bytes=0, mem_used=0]
  5.       CoalesceBatchesExec: target_batch_size=4096, metrics=[output_rows=1, elapsed_compute=17.21µs, spill_count=0, spilled_bytes=0, mem_used=0]
  6.         FilterExec: host@0 = host1, metrics=[output_rows=1, elapsed_compute=541.801µs, spill_count=0, spilled_bytes=0, mem_used=0]
  7.           CoalesceBatchesExec: target_batch_size=4096, metrics=[output_rows=3, elapsed_compute=43.004µs, spill_count=0, spilled_bytes=0, mem_used=0]
  8.             RepartitionExec: partitioning=RoundRobinBatch(10), metrics=[fetch_time=5.832958ms, repart_time=10ns, send_time=2.467µs]
  9.               RepartitionExec: partitioning=RoundRobinBatch(10), metrics=[fetch_time=386.585µs, repart_time=1ns, send_time=7.833µs]
  10.                 ExecutionPlan(PlaceHolder), metrics=[]

TODO: explains the output of ANALYZE.