STL_NESTLOOP

Analyzes nested-loop join execution steps for queries.

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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 executed.
starttime	timestamp	Time in UTC that the query started executing, 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 executing, 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 execute 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 table.

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