Thanks to Dan and Roger for the information and suggestions. I have about 100 columns (and millions of rows), but usually queries will only ask for a few columns.
I think I know a way to work around this for my case. Basically, I don't know the exact size required for my lookahead buffer (currently allocated in xFilter), so I probably need to make that size dynamic anyway rather than to allocate the maximum possible size. I don't know the size because I need to look a certain time ahead (in the timestamp column) and I don't know how many rows that will be. So if xColumn finds out that it hasn't enough lookahead data to do its computation, I should resize and read more. But at that time (unless it's the first row), I can know what the columns are that I'm going to need. So I can start out with a buffer with all the columns but few rows, and then increase the number of rows when needed and hopefully reduce it to only the interesting columns at the same time. It will require a bit of bookkeeping, but seems doable. -Steinar Roger Binns <rog...@rogerbinns.com> writes: > On 23/02/12 23:02, Steinar Midtskogen wrote: >> I know that xColumn will only get called for these columns. > > As Dan said there isn't a direct way of knowing. There is a reasonably > convenient workaround of having multiple virtual tables with different > subsets of the columns but all returning the same underlying data. > > Something else to look at are hidden columns. See section 2.1.1 in > http://www.sqlite.org/vtab.html > > With that you can make the "cheap" columns regular and the expensive ones > hidden so that the SQL query explicitly has to ask for them. In your vtab > implementation you can always start out by only getting the cheap columns > until xColumn tells you that one of the hidden/expensive ones is needed > and then go off and redo the underlying query. > > Roger > _______________________________________________ > sqlite-users mailing list > sqlite-users@sqlite.org > http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
