To improve optimizer estimates in case of skewed data distribution , histograms can be created. Prior to 12c, based on No. of distinct values (NDV) in a column two types of histograms could be created :
if no. of buckets >= NDV, frequency histogram is created and the optimizer makes accurate estimates.
If no. of buckets < NDV, height balanced histogram is created and accuracy of optimizer estimates depends on whether a key value is an endpoint or not.
The problem of optimizer mis-estimates in case of height balanced histograms is resolved to a large extent in Oracle Database 12c by introducing top-frequency and hybrid histograms which are created if no. of buckets < NDV.
I will present a webinar on “Histograms: Pre-12c and now” on Saturday, November 7th at 10:00 AM – 11:00 AM (IST) organized by All India Oracle User Group – North India Chapter.
This webinar explores Pre as well post-12c histograms while highlighting the top-frequency and hybrid histograms introduced in Oracle Database 12c.
https://plus.google.com/u/0/events/cgrgqlm5f7nuecdpjoc85d1u6eo
or
https://www.youtube.com/watch?v=xfwbDczWFXo
Hope to meet you at the webinar!!!
I wished to participate but unfortunately I found that 10 AM IST is equivalent to 4 AM in Czech Republic where I live. It would be nice to record the event and share the recording afterwards
Thanks ,
Foued
Hi Foued,
The event has been organized by All India Oracle Users Group North India Chapter (AIOUG) and will most probably be recorded.
regards
Anju
Hi Foued,
The webinar was recorded and can be accessed at :
https://www.youtube.com/watch?v=xfwbDczWFXo
and
https://plus.google.com/u/0/hangouts/onair/watch?hid=hoaevent/cgrgqlm5f7nuecdpjoc85d1u6eo&ytl=xfwbDczWFXo&wpsrc=yta
Regards
Anju