這幾天每天下午3點開始，開發人員反應Oracle慢，第一等待事件是read by other session
Top 5 Timed Foreground Events
| Event | Waits | Time(s) | Avg wait (ms) | % DB time | Wait Class |
|---|---|---|---|---|---|
| read by other session | 4,914,569 | 9,987 | 2 | 45.17 | User I/O |
| db file sequential read | 3,176,031 | 7,473 | 2 | 33.80 | User I/O |
| DB CPU | 2,128 | 9.62 | |||
| db file scattered read | 4,104,747 | 721 | 0 | 3.26 | User I/O |
| db file parallel read | 10,590 | 693 | 65 | 3.13 | User I/O |
read by other session 的定義如下：
read by other session Definition: When information is requested from the database, Oracle will first read the data from disk into the database buffer cache. If two or more sessions request the same information, the first session will read the data into the buffer cache while other sessions wait.
In previous versions this wait was classified under the “buffer busy waits” event.
However, in Oracle 10.1 and higher this wait time is now broken out into the “read by other session” wait event. Excessive waits for this event are typically due to several processes repeatedly reading the same blocks, e.g. many sessions scanning the same index or performing full-table scans on the same table. Tuning this issue is a matter of finding and eliminating this contention.
Confio concludes with a summary that “read by other session waits” are very similar to buffer busy waits
When a session waits on the “read by other session” event, it indicates a wait for another session to read the data from disk into the Oracle buffer cache.
If this happens too often the performance of the query or the entire database can suffer. Typically this is caused by contention for “hot” blocks or objects so it is imperative to find out which data is being contended for. Once that is known this document lists several alternative methods for solving the issue.
總結：兩個或者多個會話同時需要把硬盤中的對象裝載到data buffer中，當其中一個會話把對象裝入後，其他會話就處於read by other session等待狀態；這個是oracle 10g 從oracle 9i的buffer busy waits中分離出來的，也是需要一種熱塊現象導致索引熱塊的SQL如下：
select count(*) from p95169.order_info o
where o.proxyuseruuid =:1 and proxyuseruuid is not null and proxyuseruuid <> 0
and o.ORDERSTATE in ( 0, 1, 3, 2008, 2009, 3008, 3009, 5002, 5003, 5004, 5006, 7001, 7003, 7002, 7004, 7006 )
and o.ORDERCREATEDTIME >= to_char(sysdate - 15 , 'yyyyMMddHH24mmss') and o.ORDERCREATEDTIME <= to_char(sysdate , 'yyyyMMddHH24mmss')
and o.confirmstate in ( 1 ) and o.paystate in ( 1, 0 )
and o.CLINICALDATE <= to_char(sysdate, 'yyyyMMDD') and o.sourceplatid in ( 20 )通過分析執行計劃，可知它走的是以下索引：
CREATE INDEX "P95169"."IDX_ORDER_HOS_STAT_CLINICAL" ON "P95169"."ORDER_INFO"
("CLINICALDATE" DESC, "ORDERSTATE", "HOSPITALUUID", "ORDERUUID",
"RESERVETIMERANGE", "PATIENTNAME", "CONFIRMSTATE", "SOURCEPLATID", "SEEFLAG")通過觀察可知，該SQL語句where字句裏變化的字段是proxyuseruuid，因此重建一個索引，包含該字段，就可以避免索引熱塊：
CREATE INDEX "P95169"."ORDER_INFO_PROXY" ON "P95169"."ORDER_INFO" ("PROXYUSERUUID", "CLINICALDATE", "ORDERCREATEDTIME")至此，問題解決。
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