Seagate ST43401N/ND Guida di Installazione Scaricare il pdf (Pagina 62)

If Your Version is 11.5 or 11.9.x

Adaptive Server can capture query text and save an abstract plan for a query in

a new system table called

sysqueryplans. Using a rapid hashing method,

incoming SQL queries can be compared to stored query text, and if a match is

found, the saved abstract plan is used to execute the query.

For more information on abstract query plans, see the

Performance and Tuning

Guide at http://manuals.sybase.com:80/onlinebooks/group-

as/asg1250e/perf/@Generic__BookView

. For a quick overview of how to use

abstract query plans to troubleshoot performance problems after upgrade, see

the TechNote

Using and Maintaining Abstract Query Plans at

http://my.sybase.com/detail?id=1010648

Increased Optimization Time

Queries with long chains of join keys may require additional time to optimize

with Adaptive Server. If the time required to optimize such a query is

unacceptable, consider using an abstract query plan for the query.

Like Optimization Enhancements

ASE 12.0 changes the costing for like clauses that are not migrated into

SARGs, using a technique of generating more accurate cost estimates for

queries that include a leading wildcard in a

like clause.

This provides better selectivity estimates, resulting in better query plans.

Previously a like clause with a leading wild card was estimated to qualify all

rows in the column (a selectivity of 1.0) since there was no way to search for a

match. This was not the most accurate way to cost such a clause.

Example:

select ... from part, partsupp, lineitem

where l_partkey = p_partkey

and l_partkey = ps_partkey

and p_title = ’%Topographic%’

The like string is compared with histogram cell boundaries.

A match occurs when the like term is found in a cell boundary by means of a

pattern match search.

If no pattern match is found the selectivity is assumed to be 1/number of steps

in the histogram. If the default of 20 cells is used, selectivity will be 0.05 if

there is now pattern match in the histogram boundary values.

1 2 ... 57 58 59 60 61 62 63 64 65 66 67 ... 145 146

Nessun commento

Seagate ST43401N/ND Guida di Installazione Pagina 62