7.1.7 Scanned Data Monitoring

HeatWave tracks the amount of data scanned by all queries and by individual queries.

To view a cumulative total of data scanned (in MBs) by all successfully executed HeatWave queries from the time the HeatWave Cluster was last started, query the hw_data_scanned global status variable. For example:

mysql> SHOW GLOBAL STATUS LIKE 'hw_data_scanned';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| hw_data_scanned | 66    |
+-----------------+-------+

The cumulative total does not include data scanned by failed queries, queries that were not offloaded to HeatWave, or EXPLAIN queries.

The hw_data_scanned value is reset to 0 only when the HeatWave Cluster is restarted.

If a subset of HeatWave nodes go offline, HeatWave retains the cumulative total of scanned data as long as the HeatWave Cluster remains in an active state. When the HeatWave Cluster becomes fully operational and starts processing queries again, HeatWave resumes tracking the amount of data scanned, adding to the cumulative total.

To view the amount of data scanned by an individual HeatWave query or to view an estimate for the amount of data that would be scanned by a query run with EXPLAIN, run the query and query the totalBaseDataScanned field in the QKRN_TEXT column of the performance_schema.rpd_query_stats table:

mysql> SELECT query_id,
          JSON_EXTRACT(JSON_UNQUOTE(qkrn_text->'$**.sessionId'),'$[0]') AS session_id,
          JSON_EXTRACT(JSON_UNQUOTE(qkrn_text->'$**.totalBaseDataScanned'), '$[0]') AS data_scanned,
          JSON_EXTRACT(JSON_UNQUOTE(qexec_text->'$**.error'),'$[0]') AS error_message
          FROM performance_schema.rpd_query_stats;
+----------+------------+--------------+---------------+
| query_id | session_id | data_scanned | error_message |
+----------+------------+--------------+---------------+
|        1 | 8          | 66           | ""            |
+----------+------------+--------------+---------------+

The example above retrieves any error message associated with the query ID. If a query fails or was interrupted, the number of bytes scanned by the failed or interrupted query and the associated error message are returned, as shown in the following examples:

mysql> SELECT query_id,
          JSON_EXTRACT(JSON_UNQUOTE(qkrn_text->'$**.sessionId'),'$[0]') AS session_id,
          JSON_EXTRACT(JSON_UNQUOTE(qkrn_text->'$**.totalBaseDataScanned'), '$[0]') AS data_scanned,
          JSON_EXTRACT(JSON_UNQUOTE(qexec_text->'$**.error'),'$[0]') AS error_message
          FROM performance_schema.rpd_query_stats;
+----------+------------+--------------+------------------------------------------+
| query_id | session_id | data_scanned | error_message                            |
+----------+------------+--------------+------------------------------------------+
|        1 | 8          | 461          | "Operation was interrupted by the user." |
+----------+------------+--------------+------------------------------------------+

mysql> SELECT query_id,
          JSON_EXTRACT(JSON_UNQUOTE(qkrn_text->'$**.sessionId'),'$[0]') AS session_id,
          JSON_EXTRACT(JSON_UNQUOTE(qkrn_text->'$**.totalBaseDataScanned'), '$[0]') AS data_scanned,
          JSON_EXTRACT(JSON_UNQUOTE(qexec_text->'$**.error'),'$[0]') AS error_message
          FROM performance_schema.rpd_query_stats;
+----------+------------+--------------+-----------------------------------------------+
| query_id | session_id | data_scanned | error_message                                 |
+----------+------------+--------------+-----------------------------------------------+
|        1 | 8          | 987          | "Out of memory error during query execution in|
|          |            |              |  RAPID."                                      |
+----------+------------+--------------+-----------------------------------------------+