Query Plan Information and Structure
Depending on the query and database schema, EXPLAIN
output includes the following information:
- Tables referenced by the statement
- Estimated costs
- Estimated row cardinality
- Path ID, an integer that links to error messages and profiling counters so you troubleshoot performance issues more easily. For more information, see Profiling Query Plans.
- Data operations such as
SORT
,FILTER
,LIMIT
, andGROUP BY
- Projections used
- Information about statistics—for example, whether they are current or out of range
- Algorithms chosen for operations into the query, such as
HASH
/MERGE
orGROUPBY HASH
/GROUPBY PIPELINED
- Data redistribution (broadcast, segmentation) across cluster nodes
Example
In the EXPLAIN
output that follows, the optimizer processes a query in three steps, where each step identified by a unique path ID:
- 0: Limit
- 1: Sort
- 2: Storage access and filter
Note: A storage access operation can scan more than the columns in the SELECT
list— for example, columns referenced in WHERE
clause.