Faccio fatica a capire perché la stima delle righe sia terribilmente sbagliata, ecco il mio caso:
Join semplice: utilizzando SQL Server 2016 sp2 (stesso problema su 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 stima 1 riga, mentre è 107131 e sceglie di eseguire un ciclo nidificato ( collegamento al piano ). Dopo che le statistiche sono state aggiornate su CurrencyShareds, la stima va bene e viene scelto un join unione ( collegamento al nuovo piano ). Non appena viene aggiunto un solo record a CurrencyShareds, le statistiche diventano "stantie" e sql torna alla stima errata.
Non mi preoccuperei molto di questa semplice query, ma questa è solo una parte di una più grande, e questo è l'inizio di un domino ...
Perché l'aggiunta di una riga alla tabella dei 100 record provoca un danno del genere? Quando guardo l'output della traccia della stima della cardinalità, vedo questo avviso ***WARNING: badly-formed histogram ***
ma non sono riuscito a trovare altro su questo argomento.
Ecco l'output completo dalla stima della cardinalità:
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
E quando aggiorno le statistiche su CurrencyShareds la parte con modifiche "istogramma mal formato" e la cardinalità viene calcolata correttamente
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
E informazioni sulle statistiche per questo "[CurrencyShareds] .Id dalle statistiche con ID 1" con avviso sull'istogramma, che mi sembra perfetto ...
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
e informazioni per il secondo indice:
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