considering your answer, is it correct to say that SET STATISTICS
requests are now useless?

Thierry

Ann W. Harrison a écrit :

> atlioddsson wrote:
>
>> The subject pretty much sums up the question, "How to know when an
>> index has become unbalanced?"
>>
>>
>
> The short answer is that Firebird indexes basically don't go out
> of balance. If you took a really bad data structures course in
> the early dark ages of computer science, you may have seen indexes
> that grew at the bottom and developed some branches that were
> much longer than others. Professional indexes haven't worked
> like that in the last 30 years. They grow from the bottom up
> and all branches are always the same length. Until 1987 or so,
> InterBase indexes would get unbalanced in a way if you stored
> keys in ascending order (e.g. generator or timestamp based) and
> deleted the old rows. All branches would be the same length,
> but there would be large hollow areas of empty pages on the
> left hand (low) side. But in 1987 or 1988, we added index
> recompression, so that doesn't happen any more.
>
> Until Firebird, building an index in ascending order (like
> generator or timestamp keys) would create an index that was
> balanced but only half full. When an index page is about to
> overflow, Firebird splits it. Normally, half the rows go into
> the new page and half remain in the old page so new entries
> can be accommodated. But with an ascending key, new entries
> are always bigger than existing entries, so empty space in
> the old page is never used and the index is built half full.
> Now Firebird notices that a split comes at the end of the
> index, and puts only the newest entry in the new page, so
> ascending indexes are built very nearly full.
>
> However, random deletions and modification can lead to
> partly filled indexes. The tool for identifying such
> problems is gstat. It reports the fill level in index
> pages - if it is below 70%, you should probably rebuild
> the index unless the keys are very large - large keys
> don't pack well into pages.
>
>
> Regards,
>
>
> Ann
>
>
>
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