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Sampling based histogram
2
Current implementation
ANALYZE TABLE PERSISTENT FOR ALL
●
Collect histogram by doing full table scan
●
Histogram stored as equal-height 256 bytes
●
Stored in mysql database in column_stats
●
Stores all values in memory (or on disk if needed)
●
Slow
3
Improvements
●
Collect Histogram using samples
●
Avoid sorting
●
O(#rows log(#rows))
4
New Implementation
● User tells % of sampling
● We need min & max in order to build the histogram
● Equal-width Histogram
5
Sampling Steps
● First histogram
●
Sample values to get a good estimate for min & max
●
Sample again to construct histogram.
●The buckets are between min and max + 2 extra for >
max and < min
●If we know min & max ( or we already have a
histogram ) then start sampling!
●If there are too many values < min & > max, change
min & max and restart sampling

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Sampling based Histogram in MariaDB

  • 2. 2 Current implementation ANALYZE TABLE PERSISTENT FOR ALL ● Collect histogram by doing full table scan ● Histogram stored as equal-height 256 bytes ● Stored in mysql database in column_stats ● Stores all values in memory (or on disk if needed) ● Slow
  • 3. 3 Improvements ● Collect Histogram using samples ● Avoid sorting ● O(#rows log(#rows))
  • 4. 4 New Implementation ● User tells % of sampling ● We need min & max in order to build the histogram ● Equal-width Histogram
  • 5. 5 Sampling Steps ● First histogram ● Sample values to get a good estimate for min & max ● Sample again to construct histogram. ●The buckets are between min and max + 2 extra for > max and < min ●If we know min & max ( or we already have a histogram ) then start sampling! ●If there are too many values < min & > max, change min & max and restart sampling