Reintroduce :num_count variable in postgres query, as for oracle counterpart (for postgres, 15 was hardcoded)
Notes:
- :num_count comes from a parameter, defaulting to 100, instead of 15
- postgres 9.5 introduced table sampling, similar to what oracle does (but with different syntax)
- this postgres query would be portable, except for random()... maybe the random value could come from tcl...