title: KDB performance: searching first item quicky
body: <p>I have a sorted list v of around 20K items.
  I want to split it into 2 lists at the point where first v[i]>K</p>
  
  <pre><code>N:20000;
  v:asc N?100000;     / N random numbers sorted
  K:200;              / threshold
  v1:v[where v&lt;=K];   / "v&lt;=K" has O(N) complexity, "where" has O(N) too
  v2:(count v1) _ v;  / list is sorted, this holds.
  </code></pre>
  
  <p>Question: how to avoid v&lt;=200, so it does not calculate whole intermediate boolean vector of length N, in other words, does not compare values after first match found? I actually need an index to perform split at. Assume K is found close to the beginning of a list.</p>
  
  <p>This is performance-related question. (NB Ignore time spent on "asc".)</p>