How to delete random rows from a kdb table?

Question

How can I delete the first 10 rows from a kdb table? I want to delete specifically the first 10 rows returned from:

select [10] from mytable

I tried to use delete with the i index but the count of rows does not decrease:

count mytable
2201784
delete from mytable where i < 10
count mytable
2201784

Also the delete statement returns a number of rows to my Q console, not sure what that is.

Boelter

Answer 1

If you want to delete in-place from the table, you should reference it by name.

delete from `mytable where i < 10

Alternatively, reassign:

mytable:delete from mytable where i<10

When you run delete from mytable where i<10, it returns the table with the changes applied but does not apply them to mytable stored in memory.

http://code.kx.com/q/cookbook/faq/#how-do-i-delete-rows-from-a-table

The resources at http://code.kx.com answer many questions regarding day-to-day use of kdb.

Amata

Answer 2

you can use the drop operator _ on the table, and have 10 as your LHS argument mytable:10 _mytable

Exodus

Answer 3

delete from mytable where i<10

Rare

Are you sure? count md (2201784i); delete from md where i < 100; count md (2201784i). Also the delete statement using the where i<100 condition returns rows on my q console. It seems to work like a select statement. – Boelter