Hi, Min, I am not sure if you are asking a question, but that has never stopped my trying to say something.
First off, no matter whether a column is indexed or not, you can use in a select clause, e.g. "select col1, avg(col2)" or "select col2, sum(col1)", or a where clause "where col1 > 5 and col2 < 2" or where "col1 > 100 and col3 between 100 and 110". The parser will try to detect whether all the names are column names or aliases. You will see a warning message if some of them are not. When evaluating the where clause, FastBit will attempt to use an index. If the index is not available, it will try to create one. In fact, if you want to stop FastBit from trying, you will need to instruction FastBit to not index a particular column with something like index=none either in the -part.txt file or in a configuration file. Hope this helps. John On 10/29/2009 8:38 PM, Min Zhou wrote: > Thank you for your reply, John. > > I understanded that the bitmap is loading through bitwise OR , but I > do not know how to filter other columns besides col1. > Very appreciate for your help. > > > Min > > > On Thu, Oct 29, 2009 at 10:55 PM, K. John Wu <[email protected]> wrote: >> Hi, Min, >> >> FastBit does not currently support the operator 'like'. It has been >> mostly used with numerical values. For string values, the only >> expression supported is equality test. >> >> Assuming you are still interested, here is a quick description of how >> FastBit handles the following statement >> >> select * from table where col1 < 1.5 >> >> Given the basic bitmap index, where each distinct value has its own >> bitmap. when FastBit sees that col1 is involved, it will load the >> metadata of the index corresponding to col1. Using the metadata, it >> can figure out which bitmaps needs to be ORed together. Let's say >> col1 are floating-point values, and there are only three values less >> than 1.5: 0, 0.5 and 1, then the bitmaps corresponding to these three >> values marks all the rows that satisfy the condition 'col1 < 1.5.' >> FastBit needs to perform bitwise OR on these three bitmaps. In this >> process, only the necessary bitmaps are read into memory from disk. >> >> If you are interested in more information about how bitmap works in >> general, please take a look at >> <http://en.wikipedia.org/wiki/Bitmap_index>. >> >> Two fairly accessible writeups about FastBit are available at >> <http://ascr-discovery.science.doe.gov/genealogy/fastbit_print.html> >> and <http://www.scidacreview.org/0904/html/fastbit.html>. >> >> John >> >> >> On 10/28/2009 11:08 PM, Min Zhou wrote: >>> Hi guys, >>> >>> if I have a table with 5 columns(col1, col2 ... col5), they will be >>> splitted into 5 files, and col1 be indexed by fastbit. >>> can you describe how fastbit do such a query like >>> >>> select * from tbl where col1 like '%xyz%' >>> >>> I guess fastbit will load bitmaps in memory, fetch the hits by >>> bitmaps, I don't know how fastbit output other columns. >>> >>> >>> Regards, >>> Min >> _______________________________________________ >> FastBit-users mailing list >> [email protected] >> https://hpcrdm.lbl.gov/cgi-bin/mailman/listinfo/fastbit-users >> > > > _______________________________________________ FastBit-users mailing list [email protected] https://hpcrdm.lbl.gov/cgi-bin/mailman/listinfo/fastbit-users
