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How fast (or slow) a blend is depends on
cardinality (number of unique values)
of blended fields. If anything more than 10000,
the performance degrades significantly.
The type of a blended fields matters, too,
as number calcs are faster than string ones.
hope this helps.
In addition to what Yuriy said, there are a few other factors involved:
- Tableau has to query both data sources, get the results, do whatever sorting it needs to do in order to line up the domains, and then do the blend. In this architecture blends will generally be slower than joins.
- Tableau tries to make this invisible to the end user, but there are two different kinds of data blending, DB1 uses at least 3 queries to get the data, DB2*** uses at least 5, if you're triggering DB2 then that could be slowing things down due to the additional queries involved.
- Whether your primary data source supports temp tables. Here's an extreme example: In version 8, Tableau Server Published Data Sources that are extracts do not support temp tables until v9, so there have been cases where users saw a view refresh take 5-10 seconds on Desktop and 2-3 _minutes_ on Server. One of the many performance improvements in v9 is that temp table support and Server performance comes much closer to Desktop.
*** Using dimensions from the 2ndary source on any Shelf or having linking dimensions not in the view is what triggers DB2 blending in a particular view.
Thank you Jonathan for explaining.
I found blends almost "freeze" on a (v8) server all the time,
though render acceptably on a desktop.
Ought to resort to ETL to a single source and outer joins,
which works but tend to be very time consuming.
Looking (im)patiently for a v9 server release.