Seagate ST43401N/ND Información técnica descargar pdf (Pagina 77)

Migrating to Sybase Adaptive Server Enterprise 11.5 5-23

Adaptive Server Enterprise 11.5 If Your Current Version is 4.x, 10.x, or 11.0.x

select ship_to_address

from customer_orders

where last_order_date >= "01/01/1997"

GROUP BY ship_to_address

• Use the leading column of an index. If the leading column is

missing from the query, Adaptive Server can’t utilize the index.

In this example, Table A has a non-clustered index on columns

ColA, ColB, and ColC.

Coded this way, the query will not use the index:

select ColA, ColB, ColC, ColD, ColE

from tableA

where ColB = 45

and ColC = "Quality"

select ColA, ColB, ColC

from tableA

where ColC = 45

Coded this way, the query may be able to use the index because

the index may “cover” the query (the index may supply all the

data required):

select ColA, ColC

from tableA

where ColB = 45

Coded this way, the query will use the index to ﬁnd the speciﬁc

data, since each

select clause uses one or more leading columns:

select ColA, ColC, ColD, ColE

from tableA

where ColA = 45

select ColA, ColC

from tableA

where ColA = "C"

and ColB = 99

select ColA, ColC

from tableA

where ColA = "A"

and ColC = "Quality"

select ColA, ColC, ColD

from tableA

where ColA = "A"

and ColB = 401

and ColC = "Quantity"

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Seagate ST43401N/ND Información técnica Pagina 77