Próbuję zrozumieć, dlaczego szacowanie wierszy jest tak bardzo błędne, oto mój przypadek:
Proste dołączanie - przy użyciu SQL Server 2016 sp2 (ten sam problem na sp1), dbcompatiblity = 130.
select Amount_TransactionCurrency_id, CurrencyShareds.id
from CurrencyShareds
INNER JOIN annexes ON Amount_TransactionCurrency_id = CurrencyShareds.Id
option (QUERYTRACEON 3604, QUERYTRACEON 2363);
SQL szacuje 1 wiersz, podczas gdy jest to 107131 i wybiera wykonanie zagnieżdżonej pętli ( link do planu ). Po zaktualizowaniu statystyk CurrencyShareds oszacowanie jest w porządku i wybrano połączenie przez scalenie ( link do nowego planu ). Jak tylko jeden rekord zostanie dodany do CurrencyShareds, statystyki stają się „przestarzałe” i sql wraca do błędnego oszacowania.
Nie martwiłbym się tak bardzo tym prostym zapytaniem, ale to tylko część większego zapytania, a to początek domina ...
Dlaczego dodanie jednego wiersza do tabeli 100 rekordów powoduje takie uszkodzenie? Gdy patrzę na dane wyjściowe śladu szacowania liczności, widzę to ostrzeżenie, ***WARNING: badly-formed histogram ***
ale nie mogę znaleźć nic więcej na ten temat.
Oto wynik pełnej oceny z oszacowania liczności:
Begin selectivity computation
Input tree:
LogOp_Join
CStCollBaseTable(ID=1, CARD=107131 TBL: annexes)
CStCollBaseTable(ID=2, CARD=100 TBL: CurrencyShareds)
ScaOp_Comp x_cmpEq
ScaOp_Identifier QCOL: [test.MasterData].[dbo].[CurrencyShareds].Id
ScaOp_Identifier QCOL: [test.MasterData].[dbo].[Annexes].Amount_TransactionCurrency_id
Plan for computation:
CSelCalcExpressionComparedToExpression( QCOL: [test.MasterData].[dbo].[Annexes].Amount_TransactionCurrency_id x_cmpEq QCOL: [test.MasterData].[dbo].[CurrencyShareds].Id )
Loaded histogram for column QCOL: [test.MasterData].[dbo].[Annexes].Amount_TransactionCurrency_id from stats with id 7
Loaded histogram for column QCOL: [test.MasterData].[dbo].[CurrencyShareds].Id from stats with id 1 *** WARNING: badly-formed histogram ***
Selectivity: 4.59503e-018
Stats collection generated:
CStCollJoin(ID=3, CARD=1 x_jtInner)
CStCollBaseTable(ID=1, CARD=107131 TBL: annexes)
CStCollBaseTable(ID=2, CARD=100 TBL: CurrencyShareds)
End selectivity computation
Estimating distinct count in utility function
Input stats collection:
CStCollBaseTable(ID=1, CARD=107131 TBL: annexes)
Columns to distinct on:QCOL: [test.MasterData].[dbo].[Annexes].Amount_TransactionCurrency_id
Plan for computation:
CDVCPlanLeaf
0 Multi-Column Stats, 1 Single-Column Stats, 0 Guesses
Covering multi-col stats id: 7
Using ambient cardinality 107131 to combine distinct counts:
5
Combined distinct count: 5
Result of computation: 5
Estimating distinct count in utility function
Input stats collection:
CStCollBaseTable(ID=2, CARD=100 TBL: CurrencyShareds)
Columns to distinct on:QCOL: [test.MasterData].[dbo].[CurrencyShareds].Id
Plan for computation:
CDVCPlanUniqueKey
Result of computation: 100
A kiedy aktualizuję statystyki CurrencyShareds, zmienia się część ze „źle sformułowanym histogramem”, a liczność jest obliczana poprawnie
Plan for computation:
CSelCalcExpressionComparedToExpression( QCOL: [test.MasterData].[dbo].[Annexes].Amount_TransactionCurrency_id x_cmpEq QCOL: [test.MasterData].[dbo].[CurrencyShareds].Id )
Loaded histogram for column QCOL: [test.MasterData].[dbo].[Annexes].Amount_TransactionCurrency_id from stats with id 7
Loaded histogram for column QCOL: [test.MasterData].[dbo].[CurrencyShareds].Id from stats with id 1
Selectivity: 0.01
Stats collection generated:
CStCollJoin(ID=3, CARD=107131 x_jtInner)
CStCollBaseTable(ID=1, CARD=107131 TBL: annexes)
CStCollBaseTable(ID=2, CARD=100 TBL: CurrencyShareds)
End selectivity computation
I statystyki dla tego „[CurrencyShareds]. Na podstawie statystyk o id 1” z ostrzeżeniem o histogramie, który wygląda mi dobrze ...
Name Updated Rows Rows Sampled Steps Density Average key length String Index Filter Expression Unfiltered Rows Persisted Sample Percent
-------------------------------------------------------------------------------------------------------------------------------- -------------------- -------------------- -------------------- ------ ------------- ------------------ ------------ ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- -------------------- ------------------------
PK_CurrencyShareds_Id May 23 2018 10:43PM 98 98 75 1 8 NO NULL 98 0
(1 row affected)
All density Average Length Columns
------------- -------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
0,01020408 8 Id
(1 row affected)
RANGE_HI_KEY RANGE_ROWS EQ_ROWS DISTINCT_RANGE_ROWS AVG_RANGE_ROWS
-------------------- ------------- ------------- -------------------- --------------
119762190797406464 0 1 0 1
119762190797406466 1 1 1 1
119762190797406468 1 1 1 1
119762190797406470 1 1 1 1
119762190797406472 1 1 1 1
119762190797406474 1 1 1 1
119762190797406476 1 1 1 1
119762190797406478 1 1 1 1
119762190797406480 1 1 1 1
119762190797406482 1 1 1 1
119762190797406484 1 1 1 1
119762190797406486 1 1 1 1
119762190797406488 1 1 1 1
119762190797406490 1 1 1 1
119762190797406492 1 1 1 1
119762190797406494 1 1 1 1
119762190797406496 1 1 1 1
119762190797406498 1 1 1 1
119762190797406500 1 1 1 1
119762190797406502 1 1 1 1
119762190797406504 1 1 1 1
119762190797406506 1 1 1 1
119762190797406507 0 1 0 1
478531702587687680 0 1 0 1
478531702591881728 0 1 0 1
478531702591881729 0 1 0 1
478531702591881984 0 1 0 1
478531702591881985 0 1 0 1
478531702596076032 0 1 0 1
478531702596076033 0 1 0 1
478531702596076288 0 1 0 1
478531702600270336 0 1 0 1
478531702600270592 0 1 0 1
478532235583062528 0 1 0 1
478532235583062784 0 1 0 1
478532235587256832 0 1 0 1
530792464911467264 0 1 0 1
530792464924049920 0 1 0 1
530792464924050176 0 1 0 1
530792464928244224 0 1 0 1
530792464928244480 0 1 0 1
530792464932438528 0 1 0 1
530792464932438784 0 1 0 1
530792464936632832 0 1 0 1
530792464936632833 0 1 0 1
530792464936633088 0 1 0 1
530792464940827136 0 1 0 1
530792464940827392 0 1 0 1
530792464949216000 2 1 2 1
530792464953410048 0 1 0 1
530792464953410304 0 1 0 1
530792464957604352 0 1 0 1
530792464957604353 0 1 0 1
530792464957604608 0 1 0 1
530792464961798656 0 1 0 1
530792464961798912 0 1 0 1
530792464965992960 0 1 0 1
530792464965993216 0 1 0 1
530792464965993217 0 1 0 1
530792464970187264 0 1 0 1
530792464970187265 0 1 0 1
530792464970187520 0 1 0 1
530792464974381568 0 1 0 1
530792464974381824 0 1 0 1
530792464974381825 0 1 0 1
530792464978575872 0 1 0 1
530792464978575873 0 1 0 1
530792464978576128 0 1 0 1
867420708903354880 0 1 0 1
867420708903355136 0 1 0 1
867420708903355137 0 1 0 1
960876568220042240 0 1 0 1
976385263448130048 0 1 0 1
977302121709864192 0 1 0 1
977955748426318592 0 1 0 1
i informacje dla drugiego indeksu:
Name Updated Rows Rows Sampled Steps Density Average key length String Index Filter Expression Unfiltered Rows Persisted Sample Percent
-------------------------------------------------------------------------------------------------------------------------------- -------------------- -------------------- -------------------- ------ ------------- ------------------ ------------ ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- -------------------- ------------------------
IX_FK_Amount_TransactionCurrency May 21 2018 3:29PM 107204 107204 5 0 16 NO NULL 107204 0
(1 row affected)
All density Average Length Columns
------------- -------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
0,2 8 Amount_TransactionCurrency_id
9,32801E-06 16 Amount_TransactionCurrency_id, Id
(2 rows affected)
RANGE_HI_KEY RANGE_ROWS EQ_ROWS DISTINCT_RANGE_ROWS AVG_RANGE_ROWS
-------------------- ------------- ------------- -------------------- --------------
119762190797406475 0 160 0 1
119762190797406478 0 867 0 1
119762190797406481 0 106 0 1
119762190797406494 0 105742 0 1
119762190797406496 0 329 0 1