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性能陷阱:Oracle表連接中范圍比較

發(fā)布時(shí)間:  2012/8/14 17:00:56

Lately, I met a case that the range filter predicates due to wrong cardinality issue. Let’s check the following query.

  最近遇到一個(gè)由于范圍過(guò)濾導(dǎo)致錯(cuò)誤基數(shù)而引起的性能問(wèn)題。讓我們來(lái)看下面的查詢:

  The real records number is around 38,000,000.
-
 

  真實(shí)的記錄數(shù)大約3千8百萬(wàn)

  The explain plan shows 72838, optimizer think it has good filtration. So put this JOIN in the first order. Actually , it is totally wrong.

  執(zhí)行計(jì)劃顯示72838,這里優(yōu)化器認(rèn)為它有良好的過(guò)濾芯,所以把它放在一個(gè)多個(gè)表JOIN的第一位置。顯然,它完全錯(cuò)了。

  SQL> set autotrace traceonly explain;

  SQL> set linesize 999

  SQL> SELECT

  2   T.DURATIONSECSQTY TIMEINSECONDS,

  T.MONEYAMT MONEYAMOUNT,

  T.WAGEAMT WAGEAMOUNT,

  T.APPLYDTM APPLYDATE,

  T.ADJAPPLYDTM ADJUSTEDAPPLYDATE,

  T.STARTDTM,

  T.ENDDTM,

  T.HOMEACCOUNTSW

  FROM

  TKCSOWNER.WFCTOTAL     T,

  TKCSOWNER.PAYCODE1MMFLAT MP

  WHERE

  MP.EFFECTIVEDTM <= T.APPLYDTM

  AND MP.EXPIRATIONDTM > T.APPLYDTM

  AND MP.PAYCODEID = T.PAYCODEID

  /

  ---------------------------------------------------------------------

  | Id | Operation       | Name       | Rows | Bytes | Cost |

  ---------------------------------------------------------------------

  |   0 | SELECT STATEMENT   |           | 72838 | 5192K| 37450 |

  |* 1 | HASH JOIN       |           | 72838 | 5192K| 37450 |

  |   2 |   TABLE ACCESS FULL| PAYCODE1MMFLAT |   323 | 6783 |   3 |

  |   3 |   TABLE ACCESS FULL| WFCTOTAL     | 8938K|   443M| 37317 |

  Now, let me comment the range filter.

  讓我注釋到范圍條件看:

  “MP.EFFECTIVEDTM <= T.APPLYDTM

  AND MP.EXPIRATIONDTM > T.APPLYDTM”

  SQL> SELECT

  2   T.DURATIONSECSQTY TIMEINSECONDS,

  T.MONEYAMT MONEYAMOUNT,

  T.WAGEAMT WAGEAMOUNT,

  T.APPLYDTM APPLYDATE,

  T.ADJAPPLYDTM ADJUSTEDAPPLYDATE,

  T.STARTDTM,

  T.ENDDTM,

  T.HOMEACCOUNTSW

  FROM

  TKCSOWNER.WFCTOTAL     T,

  TKCSOWNER.PAYCODE1MMFLAT MP

  WHERE

  /*   MP.EFFECTIVEDTM <= T.APPLYDTM

  AND MP.EXPIRATIONDTM > T.APPLYDTM*/

  MP.PAYCODEID = T.PAYCODEID 3   4   5   6   7   8   9   10   11   12   13   14   15   16

  17 /

  Execution Plan

  ----------------------------------------------------------

  Plan hash value: 564403449

  ---------------------------------------------------------------------------

  | Id | Operation         | Name         | Rows | Bytes | Cost |

  ---------------------------------------------------------------------------

  |   0 | SELECT STATEMENT     |             |   29M| 1583M| 37405 |

  |* 1 | HASH JOIN         |             |   29M| 1583M| 37405 |

  |   2 |   INDEX FAST FULL SCAN| PK_PAYCODE1MMFLAT |   323 | 1615 |   1 |

  |   3 |   TABLE ACCESS FULL   | WFCTOTAL       | 8938K|   443M| 37317 |

  The Cardinality show 29,135,142 , it is already close to the correct value.

  基礎(chǔ)是29,135,142,已經(jīng)接近正確結(jié)果了。

  So how optimizer work out the cardinality with range filter in TABLE JOIN ?

  那么優(yōu)化器怎么出來(lái)表連接中的范圍掃描呢?

  The answer is 5%, always 5%.

  答案是5%

  29135142 * 5% * 5% = 72837.8 , This is exact equal to the result of test 1.

  So if you meet any performance issue with range filter in TBALE JOIN, I am not surprise. I think Oracle need to improve the CBO to get better support on such situation.


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