What is collect State in Teradata ? what it use and how it
works??
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Answer Posted / siri
The Optimizer plans an execution strategy for every
SQL query submitted to it. We have also seen that the
execution strategy
for any query may be subject to change depending on various
factors. For
the Optimizer to consistently choose the optimum strategy,
it must be
provided with reliable, complete, and current demographic
information
regarding all of these factors. The best way to assure that
the Optimizer
has all the information it needs to generate optimum
execution strategies
is to COLLECT STATISTICS.
-Statistics tell the Optimizer how many rows/ value there
are.
– The Optimizer uses statistics to plan the best way to
access data.
– May improve performance of complex queries and joins.
– NUSI Bit Mapping requires collected statistics.
– Helpful in accessing a column or index with uneven value
distribution.
– Stale statistics may mislead the Optimizer into poor
decisions.
– Statistics remain valid across a reconfiguration of the
system.
– COLLECT is resource intensive and should be done during
off hours.
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