STL_NESTLOOP

Analyzes nested-loop join execution steps for queries.

STL_NESTLOOP is visible to all users. Superusers can see all rows; regular users can see only their own data. For more information, see Visibility of data in system tables and views.

Note

STL_NESTLOOP only contains queries run on main provisioned clusters. It doesn't contain queries run on concurrency scaling clusters or on serverless namespaces. To access explain plans for queries run on both main clusters, concurrency scaling clusters, and serverless namespaces, we recommend that you use the SYS monitoring view SYS_QUERY_DETAIL . The data in the SYS monitoring view is formatted to be easier to use and understand.

Table columns

Column name	Data type	Description
userid	integer	ID of the user who generated the entry.
query	integer	Query ID. The query column can be used to join other system tables and views.
slice	integer	Number that identifies the slice where the query was running.
segment	integer	Number that identifies the query segment.
step	integer	Query step that ran.
starttime	timestamp	Time in UTC that the query started. Total time includes queuing and execution. with 6 digits of precision for fractional seconds. For example: 2009-06-12 11:29:19.131358.
endtime	timestamp	Time in UTC that the query finished. Total time includes queuing and execution. with 6 digits of precision for fractional seconds. For example: 2009-06-12 11:29:19.131358.
tasknum	integer	Number of the query task process that was assigned to run the step.
rows	bigint	Total number of rows that were processed.
tbl	integer	Table ID.
checksum	bigint	This information is for internal use only.

Sample queries

Because the following query neglects to join the CATEGORY table, it produces a partial Cartesian product, which is not recommended. It is shown here to illustrate a nested loop.

select count(event.eventname), event.eventname, category.catname, date.caldate
from event, category, date
where event.dateid = date.dateid
group by event.eventname, category.catname, date.caldate;

The following query shows the results from the previous query in the STL_NESTLOOP view.

select query, slice, segment as seg, step, 
datediff(msec, starttime, endtime) as duration, tasknum, rows, tbl
from stl_nestloop
where query = pg_last_query_id();

 query | slice | seg | step | duration | tasknum | rows  | tbl
-------+-------+-----+------+----------+---------+-------+-----
  6028 |     0 |   4 |    5 |       41 |      22 | 24277 | 240
  6028 |     1 |   4 |    5 |       26 |      23 | 24189 | 240
  6028 |     3 |   4 |    5 |       25 |      23 | 24376 | 240
  6028 |     2 |   4 |    5 |       54 |      22 | 23936 | 240