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Amazon Redshift
Database Developer Guide (API Version 2012-12-01)

STL_NESTLOOP

Analyzes nested-loop join execution steps for queries.

This table 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.

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.

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.

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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.

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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();
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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