• kdb+

  .Q.qp for splayed table

  Posted by darrenwsun on September 17, 2022 at 12:00 am

  The .Q.qp doc says 0b for splayed table, but on KDB 4.0 it’s actually 0. Is it a bug, or there is a new way to check if a table is splayed?

   

  q)`:./t/ set ([] v1:10 20 30; v2:1.1 2.2 3.3) 
  `:./t/ 
  
  q)l ./t 
  `t 
  
  q)0b~.Q.qp t x
  0b 
  
  q)0~.Q.qp t 
  1b

   

   

  darrenwsun replied 9 months, 2 weeks ago 2 Members · 2 Replies
• 2 Replies

• 

  sujoy

  Member

  September 17, 2022 at 12:00 am

  If you refer here: https://code.kx.com/q/basics/syscmds/#l-load-file-or-directory

  • serialized object, deserializes it into memory as variable name
  • directory of a splayed table, maps the table to variable name, without loading any columns into memory

  When you are running, l .t -> you are loading the object as t in root. Thus .Q.qp is showing t as 0

  % ls -lrth t
  total 16
  -rw-r–r–  1 sujoyrak**bleep**  staff    40B Sep 17 17:20 v1
  -rw-r–r–  1 sujoyrak**bleep**  staff    40B Sep 17 17:20 v2

  % q 
  KDB+ 4.0 2021.07.12 Copyright (C) 1993-2021 Kx Systems
  q)l .
  q)a
  ,`t
  q).Q.qp t
  0b
  q)l t
  `t
  q).Q.qp t
  0

   

• 

  darrenwsun

  Member

  September 17, 2022 at 12:00 am

  Thanks @sujoy13. What confused me is “directory of a splayed table”, which I interpreted as directory t as in your example.

  A follow-up question: what is the real runtime difference between l . and l t for this particular example? The latter form also maps the table without copying data from disk to memory. By the way t is layed out on the disk, it is a splayed table…

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