Shows the query plan of a statement.
EXPLAIN [ANALYZE] [VERBOSE] <statement>
EXPLAIN
displays the query plan that the Greenplum planner generates for the supplied statement. Query plans are a tree plan of nodes. Each node in the plan represents a single operation, such as table scan, join, aggregation or a sort.
Plans should be read from the bottom up as each node feeds rows into the node directly above it. The bottom nodes of a plan are usually table scan operations (sequential, index or bitmap index scans). If the query requires joins, aggregations, or sorts (or other operations on the raw rows) then there will be additional nodes above the scan nodes to perform these operations. The topmost plan nodes are usually the Greenplum Database motion nodes (redistribute, explicit redistribute, broadcast, or gather motions). These are the operations responsible for moving rows between the segment instances during query processing.
The output of EXPLAIN
has one line for each node in the plan tree, showing the basic node type plus the following cost estimates that the planner made for the execution of that plan node:
LIMIT
for example).WHERE
clause conditions. Ideally the top-level nodes estimate will approximate the number of rows actually returned, updated, or deleted by the query.It is important to note that the cost of an upper-level node includes the cost of all its child nodes. The topmost node of the plan has the estimated total execution cost for the plan. This is this number that the planner seeks to minimize. It is also important to realize that the cost only reflects things that the query optimizer cares about. In particular, the cost does not consider the time spent transmitting result rows to the client.
EXPLAIN ANALYZE
causes the statement to be actually executed, not only planned. The EXPLAIN ANALYZE
plan shows the actual results along with the planner's estimates. This is useful for seeing whether the planner's estimates are close to reality. In addition to the information shown in the EXPLAIN
plan, EXPLAIN ANALYZE
will show the following additional information:
The total elapsed time (in milliseconds) that it took to run the query.
The number of workers (segments) involved in a plan node operation. Only segments that return rows are counted.
The maximum number of rows returned by the segment that produced the most rows for an operation. If multiple segments produce an equal number of rows, the one with the longest time to end is the one chosen.
The segment id number of the segment that produced the most rows for an operation.
For relevant operations, the work_mem used by the operation. If work_mem
was not sufficient to perform the operation in memory, the plan will show how much data was spilled to disk and how many passes over the data were required for the lowest performing segment. For example:
Work_mem used: 64K bytes avg, 64K bytes max (seg0).
Work_mem wanted: 90K bytes avg, 90K bytes max (seg0) to abate workfile
I/O affecting 2 workers.
[seg0] pass 0: 488 groups made from 488 rows; 263 rows written to
workfile
[seg0] pass 1: 263 groups made from 263 rows
The time (in milliseconds) it took to retrieve the first row from the segment that produced the most rows, and the total time taken to retrieve all rows from that segment. The <time> to first row may be omitted if it is the same as the <time> to end.
Important: Keep in mind that the statement is actually executed when EXPLAIN ANALYZE
is used. Although EXPLAIN ANALYZE
will discard any output that a SELECT
would return, other side effects of the statement will happen as usual. If you wish to use EXPLAIN ANALYZE
on a DML statement without letting the command affect your data, use this approach:
BEGIN;
EXPLAIN ANALYZE ...;
ROLLBACK;
In order to allow the query optimizer to make reasonably informed decisions when optimizing queries, the ANALYZE
statement should be run to record statistics about the distribution of data within the table. If you have not done this (or if the statistical distribution of the data in the table has changed significantly since the last time ANALYZE
was run), the estimated costs are unlikely to conform to the real properties of the query, and consequently an inferior query plan may be chosen.
An SQL statement that is run during the execution of an EXPLAIN ANALYZE
command is excluded from Greenplum Database resource queues.
For more information about query profiling, see "Query Profiling" in the Greenplum Database Administrator Guide. For more information about resource queues, see "Resource Management with Resource Queues" in the Greenplum Database Administrator Guide.
To illustrate how to read an EXPLAIN
query plan, consider the following example for a very simple query:
EXPLAIN SELECT * FROM names WHERE name = 'Joelle';
QUERY PLAN
------------------------------------------------------------
Gather Motion 2:1 (slice1) (cost=0.00..20.88 rows=1 width=13)
-> Seq Scan on 'names' (cost=0.00..20.88 rows=1 width=13)
Filter: name::text ~~ 'Joelle'::text
If we read the plan from the bottom up, the query optimizer starts by doing a sequential scan of the names
table. Notice that the WHERE
clause is being applied as a filter condition. This means that the scan operation checks the condition for each row it scans, and outputs only the ones that pass the condition.
The results of the scan operation are passed up to a gather motion operation. In Greenplum Database, a gather motion is when segments send rows up to the master. In this case we have 2 segment instances sending to 1 master instance (2:1). This operation is working on slice1
of the parallel query execution plan. In Greenplum Database a query plan is divided into slices so that portions of the query plan can be worked on in parallel by the segments.
The estimated startup cost for this plan is 00.00
(no cost) and a total cost of 20.88
disk page fetches. The planner is estimating that this query will return one row.
There is no EXPLAIN
statement defined in the SQL standard.
Parent topic: SQL Command Reference