KX Community

Find answers, ask questions, and connect with our KX Community around the world.

Home Forums KX Solutions RDB/IHDB/HDB – What is the best way of a copy of the values of an in-memory table?

  • RDB/IHDB/HDB – What is the best way of a copy of the values of an in-memory table?

    Posted by Laura on July 6, 2021 at 12:00 am

    Assume I have an RDB (or IHDB / HDB) that has access to a schema called people.

    Here is some data you can put on people:

    ([] sym: `ada`maggie; name: `ada`maggie; age 33 25)

    Assume also that I have a function called doSomething that receives the table name as it is argument and performs a series of operations, including an xkey at some point:

    doSomething:{[tableName] // blablablah some code r: `sym xkey value tableName; // some more code }

    q does not like to perform the xkey directly on value tableName, so the solution I currently have for this is performing a select:

    doSomething:{tableName //blablablah some code r: `sym xkey ?[value tableName; (); 0b; ()]; //blablablah some more code }

    My question is: is there a better way to get around that or is my solution fine?


    Laura replied 2 months ago 2 Members · 3 Replies
  • 3 Replies
  • Laura

    July 6, 2021 at 12:00 am

    Hi Adriano,

    We’ve had a look at your example and for in-memory tables your first solution appears to work perfectly as long as you are passing a reference into your function.

    for example:




    However if you were working with an on-disk table rather than an in-memory table your solution could be simplified to


    q)r:`sym xkey select from tableName


    using the select function in order to pull your table into memory first.

    Your functional form solution works fine however most people will use q-SQL as it is a more readable format. A where clause can be used alongside the select query to avoid pulling in a large amount of data from disk.



  • dcrossey

    July 6, 2021 at 12:00 am

    Hi Adriano,

    By “q does not like to perform the xkey directly on value table”, do you mean as follows:


    q)`name xkey value people


    [0] `name xkey value people ^


    Here, we are referring to the table with pass-by-value and q throws a type error. You don’t need to use ‘value’ if you are referring to a table in this manner (note the lack of backtick).

    If you used the ` people with ‘value’ this would have worked as expected (pass-by-reference) :


    q)`name xkey value `people 
    name  | age sym 
    ------| ---------- 
    ada   | 33 ada 
    maggie| 25 maggie


    In general, I would advise the following syntax for general usage (pass-by-value) :


    people:`name xkey people;


    Otherwise using pass-by-reference:


    people:`name xkey value `people;


    Hope this helps.


  • Laura

    July 7, 2021 at 12:00 am

    Hi Kate and David,

    Both of your tips were great, thank you very much

    It seems I will have to adopt the solution for on-disk tables that Kate suggested because the function doSomething is routed to the IHDB and performs its operations on on-disk tables.

    So, when people is an on-disk table, if i try pass by value:

    people: `name xkey people

    then the IHDB throws a int error message
    if i do:

    `name xkey select from people

    then the IHDB is happy. Since I won’t dare to change the key of my table on the IHDB, it looks like pulling the values to the memory first is the safest option.

    Thanks again!

Log in to reply.