What is the need of a flip of splay or partitioned table ?

[megan_mcp]

Hi @newstudent2017

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)<br>: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)<br>:db/2001.01.01/t/
q):db/2001.01.02/t/ set ([]a:3 4)<br>: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