今天處理客戶數據庫問題的時候，發現該數據庫有大量的read by other session等待事件，藉此記錄下。
首先根據文檔，這個事件的解釋如下：
When informationis requested from the database, Oracle will first read the data from disk intothe database buffer cache. If two or more sessions request the sameinformation, the first session will read the data into the buffer cache whileother sessions wait. In previous versions this wait was classified under the"buffer busy waits" event. However, in Oracle 10.1 and higher thiswait time is now broken out into the "read by other session" waitevent. Excessive waits for this event are typically due to several processesrepeatedly reading the same blocks, e.g. many sessions scanning the same indexor performing full table scans on the same table. Tuning this issue is a matterof finding and eliminating this contention.
當請求一個數據時，如果buffer cache沒有該數據時，Oracle會從磁盤將數據讀入buffer cache。
如果有兩個或者多個session 請求相同的信息，那麼第一個session 會將這個信息讀入buffer cache，其他的session 就會出現等待。
在10.1之前，該等待事件歸類爲bufferbusy waits，在Oracle 10.1之後，單獨將該事件拿出並命令爲read by other session。
一般來說出現這種等待事件是因爲多個進程重複的讀取相同的blocks，比如一些session 掃描相同的index或者在相同的block上執行full table scan。
解決這個等待事件最好是找到並優化相關的SQL語句。
read by other session 等待的出現也說明數據庫存在熱塊問題，所以該等待事件通常會有db file sequential read或db file scattered read 同時出現。
來看看今天遇到的情況：
在查看數據庫等待事件的時候，發現有大量的read other session和db file scattered read事件
- select sid,
- serial#,
- username,
- status,
- machine,
- program,
- sql_hash_value,
- sql_id,
- to_char(logon_time,'yyyy-mm-dd hh24:mi:ss'),
- event,
- p1,
- p2
- from v$session
- where event in ('read by other session', 'db file scattered read');
- SID SERIAL# USERNAME STATUS MACHINE PROGRAM SQL_HASH_VALUE SQL_ID TO_CHAR(LOGON_TIME,'YYYY-MM-DD EVENT P1 P2
- ---------- ---------- ----------- -------- --------------- ----------------------------- -------------- ------------- ------------------------------ ----------------------------- ---------- ----------
- 347 822 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:44:20 read by other session 9 103885
- 348 1558 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:34:45 read by other session 9 171789
- 349 1879 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:17:03 db file scattered read 9 610910
- 350 1241 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:46:19 read by other session 9 103885
- 351 1067 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:44:50 db file scattered read 9 103885
- 352 3296 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:35:19 read by other session 9 171789
- 353 391 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:21:49 db file scattered read 9 441841
- 354 616 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:23:18 db file scattered read 9 391294
- 355 681 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:25:13 read by other session 9 279803
- 356 712 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:33:17 read by other session 9 171789
- 357 510 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:45:17 read by other session 9 103885
- 358 1625 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:30:38 read by other session 9 171789
- 359 956 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:20:13 db file scattered read 9 568327
- 360 837 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:16:22 db file scattered read 9 624695
- 364 10488 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:17:30 db file scattered read 9 590289
- 370 3993 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:20:37 db file scattered read 9 451617
- 376 3258 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:49:47 db file scattered read 9 69053
- 377 4779 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:44:12 read by other session 9 103885
- 380 21129 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:15:39 db file scattered read 9 668803
- 381 2716 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:34:07 read by other session 9 171789
- 385 1617 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:27:59 read by other session 9 171789
- 389 1681 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:16:07 db file scattered read 9 643364
- 391 6657 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:26:49 db file scattered read 9 234271
- 392 43902 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:30:23 read by other session 9 171789
- 395 7003 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:15:15 db file scattered read 9 719929
- 398 5246 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:33:25 read by other session 9 171789
- 399 9635 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:15:39 db file scattered read 9 672689
- 410 5588 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:17:05 db file scattered read 9 604003
- 412 980 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:31:20 read by other session 9 171789
- 415 4591 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:20:13 db file scattered read 9 556541
- 416 6649 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:33:33 read by other session 9 171789
- 418 21354 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:27:21 db file scattered read 9 183001
- 419 43412 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:17:26 db file scattered read 9 592652
- 420 3669 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:19:26 db file scattered read 9 581417
- 423 2586 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:33:25 read by other session 9 171789
- 430 27397 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:31:50 read by other session 9 171789
- 432 7621 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:19:39 db file scattered read 9 534361
- 435 3845 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:29:16 read by other session 9 171789
- 439 3293 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:44:09 read by other session 9 103885
- 440 16800 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:43:40 read by other session 9 103885
- 446 18697 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:18:18 db file scattered read 9 584413
- 451 4734 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:45:40 read by other session 9 103885
- 452 2594 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:27:05 db file scattered read 9 210845
- 453 9508 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:16:22 db file scattered read 9 615554
- 458 6139 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:44:31 read by other session 9 103885
- 460 18012 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:44:08 read by other session 9 103885
- 461 3224 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:14:26 db file scattered read 9 724229
- 465 4371 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:34:18 read by other session 9 171789
- 466 4650 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:20:22 read by other session 9 534361
- 468 8735 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:25:22 db file scattered read 9 268194
- 469 14362 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:27:05 db file scattered read 9 202463
- 471 7194 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:44:07 read by other session 9 103885
- 474 2533 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:25:37 db file scattered read 9 248817
- 478 3757 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:46:43 read by other session 9 103885
- 485 4438 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:26:28 db file scattered read 9 236349
- 488 7859 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:26:19 read by other session 9 239613
- 489 8827 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:45:17 read by other session 9 103885
- 491 6996 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:24:16 db file scattered read 9 293921
- 492 21059 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:51:42 read by other session 9 69053
- 493 7681 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:30:42 read by other session 9 171789
- 497 4636 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:28:42 read by other session 9 171789
- 500 4062 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:48:06 read by other session 9 103885
- 501 9340 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:26:19 db file scattered read 9 239613
- 504 9746 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:33:57 read by other session 9 171789
- 506 4258 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:17:05 db file scattered read 9 605955
- 507 3436 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:17:59 db file scattered read 9 584829
- 509 3990 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:15:37 db file scattered read 9 689377
- 511 5592 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:48:47 read by other session 9 69053
- 515 2380 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:33:11 read by other session 9 171789
- 516 5049 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:23:42 db file scattered read 9 375192
- 517 11081 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:16:49 read by other session 9 612067
- 518 19019 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:19:16 db file scattered read 9 582651
- 519 2947 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:44:03 read by other session 9 103885
- 520 44555 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:19:39 db file scattered read 9 573202
- 523 32606 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:43:40 read by other session 9 103885
- 524 14352 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:25:40 read by other session 9 248817
- 525 49196 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:33:43 read by other session 9 171789
- 526 2815 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:22:03 db file scattered read 9 432421
- 527 7558 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:20:40 db file scattered read 9 447233
- 528 19934 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:18:42 db file scattered read 9 583660
- 532 12000 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:15:23 db file scattered read 9 703321
- 535 19291 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:15:40 db file scattered read 9 651921
- 537 5808 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:16:28 db file scattered read 9 612067
- 539 5824 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:44:08 read by other session 9 103885
- 542 22756 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:44:08 read by other session 9 103885
- 543 28090 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:21:19 db file scattered read 9 446657
- 546 25606 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:21:19 db file scattered read 9 443777
- 547 21390 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:31:28 read by other session 9 171789
- 548 31131 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:22:59 db file scattered read 9 392686
- 557 20900 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:22:05 db file scattered read 9 420589
- 558 7787 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:21:28 db file scattered read 9 442321
- 560 49814 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:45:14 read by other session 9 103885
- 561 33975 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:28:18 read by other session 9 171789
- 603 708 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:33:50 read by other session 9 171789
- 645 24200 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:22:05 db file scattered read 9 419926
- 663 8601 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:23:42 db file scattered read 9 376348
- 669 42540 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:44:08 read by other session 9 103885
- 670 355 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:31:20 db file scattered read 9 171789
- 671 5023 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:33:57 read by other session 9 171789
- 676 572 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:14:39 db file scattered read 9 723209
- 677 5142 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:16:26 db file scattered read 9 615415
- 687 8508 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:15:15 db file scattered read 9 711833
- 688 565 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:32:03 read by other session 9 171993
- 700 20809 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:25:13 db file scattered read 9 279835
- 702 3931 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:33:21 read by other session 9 171993
- 705 3521 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:16:00 db file scattered read 9 649425
- 772 1005 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:27:21 db file scattered read 9 189930
- 797 1081 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:47:35 read by other session 9 104045
- 806 4809 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:18:42 read by other session 9 584461
- 819 32486 SYH123 ACTIVE LENOVO-EWJS4146 300319264 187mwcn8yd0j0 2013-02-01 11:28:43 read by other session 9 171993
- 839 26168 SYH123 ACTIVE LENOVO-EWJS4146 2307979633 bj3rk3a4t1ybj 2013-02-01 11:27:03 db file scattered read 9 232669
進一步查看read other session和db file scattered read事件相關語句的SQL_ID
- SQL> select distinct sql_id
- 2 from v$session
- 3 where event in ('read by other session', 'db file scattered read');
- SQL_ID
- -------------
- bj3rk3a4t1ybj
- 187mwcn8yd0j0
發現所有的read other session和db file scattered read事件都集中在上面兩個SQL語句上。
在查看read other session的參數情況：
- SQL> select name,parameter1,parameter2,wait_class from v$event_name where name='read by other session';
- NAME PARAMETER1 PARAMETER2 WAIT_CLASS
- ------------------------------ ---------- ---------- --------------------
- read by other session file# block# User I/O
- SQL>
可以看p1代表了file#，p2代表block#
查看這些read other session所訪問的對象
- SQL> select t.owner,t.segment_name,t.segment_type from dba_extents t where t.file_id = 9 and 103885 between t.block_id and t.block_id+t.blocks
- 2 /
- OWNER SEGMENT_NAME SEGMENT_TYPE
- ------------------------------ ---------------------------------------- ------------------
- SCPEND TF_TASK_TASKINFOHISTB TABLE
- SQL>
- SQL>
- SQL> select t.owner, t.segment_name, t.segment_type
- 2 from dba_extents t
- 3 where t.file_id = 9
- 4 and 69053 between t.block_id and t.block_id + t.blocks;
- OWNER SEGMENT_NAME SEGMENT_TYPE
- ------------------------------ ---------------------------------------- ------------------
- SCPEND TF_TASK_TASKINFOHISTB TABLE
- SQL>
- SQL>
- SQL> select t.owner, t.segment_name, t.segment_type
- 2 from dba_extents t
- 3 where t.file_id = 9
- 4 and 590289 between t.block_id and t.block_id + t.blocks;
- OWNER SEGMENT_NAME SEGMENT_TYPE
- ------------------------------ ---------------------------------------- ------------------
- SCPEND TF_TASK_TASKINFOHISTB TABLE
發現都是訪問的同一個表。
再來看看該SQL的執行計劃：
可以看到相關SQL確實在對該表做全表掃描的操作，耗時達到22分鐘。
後續操作就是對該SQL進行優化