The SHOW FULL TABLE SCANS statement lists recent queries for which CockroachDB performed a full table scan during query execution.
Limiting the number of queries that require full table scans can help you optimize query execution performance. For more information on query performance optimization, see Optimize Statement Performance and SQL Tuning with EXPLAIN.
Syntax
Required privileges
The admin role is required to run SHOW FULL TABLE SCANS.
Examples
To follow along, run cockroach demo to start a temporary, in-memory cluster with the sample movr dataset preloaded:
$ cockroach demo
Now, suppose that you want to query the rides table for all rides that cost above 90 (i.e., WHERE revenue > 90):
> SELECT * FROM rides WHERE revenue > 90;
id | city | vehicle_city | rider_id | vehicle_id | start_address | end_address | start_time | end_time | revenue
---------------------------------------+---------------+---------------+--------------------------------------+--------------------------------------+-----------------------------------+------------------------------------+---------------------+---------------------+----------
b4bc6a7e-f9db-4000-8000-000000000161 | amsterdam | amsterdam | ae147ae1-47ae-4800-8000-000000000022 | bbbbbbbb-bbbb-4800-8000-00000000000b | 73310 Young Harbor | 31482 Omar Street | 2018-12-13 03:04:05 | 2018-12-13 07:04:05 | 92.00
b9db22d0-e560-4000-8000-00000000016b | amsterdam | amsterdam | c28f5c28-f5c2-4000-8000-000000000026 | aaaaaaaa-aaaa-4800-8000-00000000000a | 66748 Carroll Ports Apt. 42 | 65413 Miller Point Suite 62 | 2018-12-08 03:04:05 | 2018-12-10 05:04:05 | 98.00
...
4fdf3b64-5a1c-4c00-8000-00000000009c | washington dc | washington dc | 47ae147a-e147-4000-8000-00000000000e | 44444444-4444-4400-8000-000000000004 | 44086 Barbara Ville | 88493 Michael Flat Apt. 97 | 2018-12-27 03:04:05 | 2018-12-28 05:04:05 | 91.00
(51 rows)
This SELECT statement requires a full table scan at execution. As a result, the query will show up in the SHOW FULL TABLE SCANS output, with all of the other queries that performed full table scans:
> WITH x AS (SHOW FULL TABLE SCANS) SELECT * FROM x WHERE query LIKE 'SELECT * FROM rides WHERE revenue > %';
query | count | rows_read_avg | bytes_read_avg | service_lat_avg | contention_time_avg | max_mem_usage_avg | network_bytes_avg | max_retries
----------------------------------------+-------+---------------+----------------+-----------------+---------------------+-------------------+-------------------+--------------
SELECT * FROM rides WHERE revenue > _ | 1 | 500 | 88497 | 0.00456 | 0 | 450560 | 0 | 0
(1 row)
To limit the number of rows scanned by SELECT queries that filter on the revenue column, you can add a secondary index to the rides table, on the revenue column:
> CREATE INDEX ON rides (revenue);
Now, if you execute a similar query, the query will not perform a full table scan.
> SELECT * FROM rides WHERE revenue < 10;
id | city | vehicle_city | rider_id | vehicle_id | start_address | end_address | start_time | end_time | revenue
---------------------------------------+---------------+---------------+--------------------------------------+--------------------------------------+---------------------------------+---------------------------------+---------------------+---------------------+----------
ac083126-e978-4800-8000-000000000150 | amsterdam | amsterdam | c28f5c28-f5c2-4000-8000-000000000026 | aaaaaaaa-aaaa-4800-8000-00000000000a | 50217 Victoria Fields Apt. 44 | 56217 Wilson Spring | 2018-12-07 03:04:05 | 2018-12-07 10:04:05 | 9.00
ae147ae1-47ae-4800-8000-000000000154 | amsterdam | amsterdam | bd70a3d7-0a3d-4000-8000-000000000025 | aaaaaaaa-aaaa-4800-8000-00000000000a | 63503 Lisa Summit Suite 28 | 26800 Brown Station | 2018-12-25 03:04:05 | 2018-12-26 22:04:05 | 0.00
...
(32 rows)
> WITH x AS (SHOW FULL TABLE SCANS) SELECT * FROM x WHERE query LIKE 'SELECT * FROM rides WHERE revenue < %';
query | count | rows_read_avg | bytes_read_avg | service_lat_avg | contention_time_avg | max_mem_usage_avg | network_bytes_avg | max_retries
--------+-------+---------------+----------------+-----------------+---------------------+-------------------+-------------------+--------------
(0 rows)
See also
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