The execution of an SQL statement goes through two phases: compilation and execution. In the compilation phase, the optimizer performs operations such as lexical analysis, syntax analysis, syntax optimization, and code generation to generate an execution plan. The plan is then submitted to the executor for execution, which returns the results. The process of parsing an SQL statement and generating an execution plan is called hard parsing. Hard parsing consumes resources and increases SQL latency. To avoid the overhead of hard parsing for each request, execution plans are cached in the plan cache. When the optimizer receives a subsequent SQL statement of the same type, it first checks the plan cache for a matching plan. If a matching plan is found in the cache, it is retrieved and submitted to the executor for execution, which returns the results. This process is called soft parsing. Therefore, whether an execution plan is cached in the plan cache and whether the plan is optimal are key factors in performance optimization.
The optimizer may generate non-optimal execution plans due to various reasons, such as missing indexes, outdated statistics, and account privileges. Non-optimal execution plans lead to a high number of logical reads and large amounts of scanned data, which increases latency and reduces throughput.
You can determine whether a performance issue is an SQL issue and identify suspicious SQL statements by querying the V$OB_SQL_AUDIT view. You can then determine whether there is an issue with the execution plan by querying the relevant plan views. If the execution plan is suboptimal, you can bind it to a SQL statement by using the CREATE OUTLINE statement or recover from the suboptimal plan by scaling up or throttling back.
Note
For more information about the V$OB_SQL_AUDIT view, see V$OB_SQL_AUDIT(Oracle mode) and V$OB_SQL_AUDIT(MySQL mode).
For example, the following example shows how to obtain TopSQL statements and determine whether SQL issues exist. The SQL statement with the SQL_ID of 2705182A6EAB699CEC8E59DA80710B64 is the top SQL statement that consumed the most tenant resources. It also has a high number of logical reads. This is a SQL statement that needs to be optimized.
obclient>
select SQL_ID,
avg(ELAPSED_TIME),
avg(QUEUE_TIME),
avg(ROW_CACHE_HIT + BLOOM_FILTER_CACHE_HIT + BLOCK_CACHE_HIT + DISK_READS) avg_logical_read,
avg(execute_time) avg_exec_time,
count(*) cnt,
avg(execute_time - TOTAL_WAIT_TIME_MICRO ) avg_cpu_time,
avg( TOTAL_WAIT_TIME_MICRO ) avg_wait_time,
WAIT_CLASS,
avg(retry_cnt)
from v$OB_SQL_AUDIT
group by 1
order by avg_exec_time * cnt desc limit 10;
+----------------------------------+-------------------+-----------------+------------------+---------------+--------+--------------+---------------+------------+----------------+
| SQL_ID | avg(ELAPSED_TIME) | avg(QUEUE_TIME) | avg_logical_read | avg_exec_time | cnt | avg_cpu_time | avg_wait_time | WAIT_CLASS | avg(retry_cnt) |
+----------------------------------+-------------------+-----------------+------------------+---------------+--------+--------------+---------------+------------+----------------+
| 2705182A6EAB699CEC8E59DA80710B64 | 54976.9269 | 43.8605 | 17664.2727 | 54821.5828 | 11759 | 54821.5828 | 0.0000 | OTHER | 0.0000 |
| 32AB97A0126F566064F84DDDF4936F82 | 1520.9832 | 380.7903 | 63.7847 | 789.6781 | 63632 | 789.6781 | 0.0000 | OTHER | 0.0000 |
| A5F514E873BE9D1F9A339D0DA7481D69 | 44032.5553 | 44.5149 | 8943.7834 | 43878.1405 | 1039 | 43878.1405 | 0.0000 | OTHER | 0.0000 |
| 31FD78420DB07C11C8E3154F1658D237 | 7769857.0000 | 35.7500 | 399020.7500 | 7769682.7500 | 4 | 7769682.7500 | 0.0000 | NETWORK | 1.0000 |
| C48AEE941D985D8DEB66892228D5E845 | 8528.6227 | 0.0000 | 0.0000 | 8450.4047 | 1601 | 8450.4047 | 0.0000 | OTHER | 0.0000 |
| 101B7B79DFA9AE801BEE4F1A234AD294 | 158.2296 | 41.7211 | 0.0000 | 46.0345 | 286758 | 46.0345 | 0.0000 | OTHER | 0.0000 |
| 1D0BA376E273B9D622641124D8C59264 | 1774.5924 | 0.0049 | 0.0000 | 1737.4885 | 5081 | 1737.4885 | 0.0000 | OTHER | 0.0000 |
| 64CF75576816DB5614F3D5B1F35B1472 | 1801.8767 | 747.0343 | 0.0000 | 827.1674 | 10340 | 827.1674 | 0.0000 | OTHER | 0.0000 |
| 23D1C653347BA469396896AD9B20DCA1 | 5564.9419 | 0.0000 | 0.0000 | 5478.2228 | 1257 | 5478.2228 | 0.0000 | OTHER | 0.0000 |
| FA4F493FA5CE2DCC64F51CF3754F96C6 | 2478.3956 | 378.7557 | 3.1040 | 1731.1802 | 3357 | 1731.1802 | 0.0000 | OTHER | 0.0000 |
+----------------------------------+-------------------+-----------------+------------------+---------------+--------+--------------+---------------+------------+----------------+
10 rows in set (1.34 sec)