Hi all, as far as I aware, there is no command to delete some records in HDB, I have to load all records to memory, modify it, and save them back, is that correct?
You can load all records into memory or modify the splayed column files directly. It's a bit fiddly as for this you need to know the row index of the data you're no longer interested in. Here's an example:
// some random data n:1000; mytbl:([]id:n?0Ng;name:n?`foo`bar`baz;sz:n?50;px:n?100.0); .Q.dpft[`:db;2015.07.25;`name;`mytbl];
You can delete records without loading the whole table into memory, but it's a non-atomic operation so if you do choose to do it I'd recommend copying the partition into a staging directory, doing the delete and then copying it back into your main hdb directory.
q)t:([]sym:`a`b`c`d;a:1 2 3 4;b:10 20 30 40)
q).Q.dpft[`:.;.z.D;`sym;`t]
`t
q)\l .
q)t
date sym a b
-------------------
2015.07.26 a 1 10
2015.07.26 b 2 20
2015.07.26 c 3 30
2015.07.26 d 4 40
//Let's say we want to delete where sym=`b. Get the index of the rows you want to remove
q)inds:exec x from select i from t where sym=`b
q)inds
,1
//Then figure out the indices of the rows that you don't want to remove
q)keep:(exec til first x from select count i from t where date=2015.07.26) except inds
q)keep
0 2 3
//Now iterate over each column in the parition, keeping the relevant indices and discarding the ones we want to remove
q).[;();@;keep] each `:./2015.07.26/t/sym`:./2015.07.26/t/a`:./2015.07.26/t/b
