Vicențiu Ciorbaru
f0773b7842
Introduce analyze_sample_percentage variable
...
The variable controls the amount of sampling analyze table performs.
If ANALYZE table with histogram collection is too slow, one can reduce the
time taken by setting analyze_sample_percentage to a lower value of the
total number of rows.
Setting it to 0 will use a formula to compute how many rows to sample:
The number of rows collected is capped to a minimum of 50000 and
increases logarithmically with a coffecient of 4096. The coffecient is
chosen so that we expect an error of less than 3% in our estimations
according to the paper:
"Random Sampling for Histogram Construction: How much is enough?”
– Surajit Chaudhuri, Rajeev Motwani, Vivek Narasayya, ACM SIGMOD, 1998.
The drawback of sampling is that avg_frequency number is computed
imprecisely and will yeild a smaller number than the real one.
2019-02-19 12:01:21 +02:00
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