Dennis Cote wrote:
Stephen Toney wrote:
Meta-question: this is the second time I've asked this question. The
first was about a month ago and got not a single reply. Is there
something wrong with my postings? Or is this just not an interesting
topic?
Stephen,
There is nothing wrong with your question. In fact it was very clearly
stated.
I just don't have any answers for you. I suspect that others are in the
same boat.
It is normal for an index creation operation to take some time since it
is inserting index records into a btree in random order. It involves
many updates to pages throughout the index.
It does seem strange that you are seeing such different times for the
two cases (single column vs compound index). Are you sure about the
times you posted? Were they indexing the exact same table? Were both the
indexes created after the fill operation during your timing tests (ie.
fill + index(single) and then fill + index(compound)) and not one after
the other (i.e. fill + index(single) + index(compound))?
Can you supply sample data if someone wants to try some test of their
own? It wouldn't have to be the full data set you are using. We could
use a subset to get relative timings in the seconds range rather than
minutes (This assumes that you are not running into some cache size
problems that slow down the larger data set disproportionately).
Dennis Cote
I suspect that the timing difference is due to page overflows. I did
only a cursory browse of the B-Tree code but it is just a guess. A test
would be to make a simple table with two adjacent integer columns and
time raising an index on one column and on both. If the times are
comparable the speed difference reported in this thread is a page
overflow artifact.
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