Commit fd422c954e aimed to fix
issue #3803. In that issue, if a query SELECTed only certain columns but
did filtering (ALLOW FILTERING) over other unselected columns, the filtering
didn't work. The fix involved adding the columns being filtered to the set
of columns we read from disk, so they can be filtered.
But that commit included an optimization: If you have clustering keys
c1 and c2, and the query asks for a specific partition key and c1 < 3 and
c2 > 3, the "c1 < 3" part does NOT need to be filtered because it is already
done as a slice (a contiguous read from disk). The committed code erroneously
concluded that both c1 and c2 don't need to be filtered, which was wrong
(c2 *does* need to be read and filtered).
In this patch, we fix this optimization. Previously, we used the "prefix
length", which in the above example was 2 (both c1 and c2 were filtered)
but we need a new and more elaborate function,
num_prefix_columns_that_need_not_be_filtered(), to determine we can only
skip filtering of 1 (c1) and cannot skip the second.
Fixes#4121. This patch also adds a unit test to confirm this.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20190123131212.6269-1-nyh@scylladb.com>
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