https://learninghub.kx.com/forums/topic/what-is-the-need-of-a-flip-of-splay-or-partitioned-table
https://code.kx.com/q/ref/flip-splayed/
What is the need of a flip of splay or partitioned table ?
Possible answers:
1. Brute way of doing it would be to load each column (and each date) into memory and do a flip. kdb is providing this as a service ?
2. What does it have to do with .Q.qp ?
This operation is used internally by kdb+ to represent the flip of a memory-mapped splayed table. When loading a database with \l, the tables in the database are added to the root namespace using this operation.
x!y ![x;y]
The result must be flipped in order to use it as a table. After doing that, select statements will operate on the on-disk table. Many operations (including certain overloads of select) will throw a par error when used on this table.
Examples:
q):db/t/ set ([]a:1 2):db/t/
q)\l db
q).Q.s1 t
"+(,a)!:./t/"
It is possible to manually create this representation:
q)enlist[a]!:./t/
(,a)!:./t/
q)flip enlist[a]!:./t/
a
-
1
2
The equivalent for a partitioned table:
q):db/2001.01.01/t/ set ([]a:1 2):db/2001.01.01/t/
q):db/2001.01.02/t/ set ([]a:3 4):db/2001.01.02/t/
q)\l db
q).Q.s1 t
"+(,a)!t"
q)enlist[a]!t
(,a)!t
q)flip enlist[a]!t
date a
------------
2001.01.01 1
2001.01.01 2
2001.01.02 3
2001.01.02 4
If the specified table does not exist on disk, the expression remains unresolved and any attempt to select from it fails:
q)flip enlist[a]!:./s/
+(,a)!:./s/
q)select from flip enlist[a]!:./s/
'./s/a. OS reports: No such file or directory
[0] select from flip enlist[a]!:./s/
q)flip enlist[a]!s
+(,a)!s
q)select from flip enlist[a]!s
's
[0] select from flip enlist[a]!s
^
We are adjusting our documentation to make this clearer in future.
If you have any further questions, please let me know!
Thanks,
Megan