Seagate ST43401N/ND Informações Técnicas descarregar pdf (Página 71)

Migrating to Sybase Adaptive Server Enterprise 11.5 5-17

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

Subquery Processing Changes

Changes in subquery processing are described in the following

sections:

• Expression Subqueries 5-17

• No set dup in subquery 5-17

• union Limitations 5-18

• Subqueries and NULL Results 5-18

• No Subqueries in Updatable Cursors 5-18

Expression Subqueries

Expression subqueries may be slower in release 11.x than 10.x where:

• The outer table is very large and has few duplicate correlation

values.

• The inner table is small.

• The subquery contains an aggregate.

The optimizer will not ﬂatten this type of query to be processed as a

join. Such a query might look like this:

select * from huge_table where col_x=

(select sum(col_a) from tiny_table

where col_b = huge_table.col_y)

To get faster results, you can reformulate the query to mimic the

behavior of release 10.x, as demonstrated in this example:

select huge_table.col_y, s=sum(col_a)

into #t

from huge_table, tiny_table

where col_b=huge_table.col_y

group by huge_table.col_y

select huge_table.*

from huge_table, #t

where col_x=#t.s

and huge_table.col_y=#t.col_y

No set dup in subquery

The change in the

set dup in subquery command does not affect systems

being upgraded from 4.x, as this command was introduced in

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Seagate ST43401N/ND Informações Técnicas Página 71