a question from KDB beginner

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1

Hi everyone,Recently I started to learn KDB, and I saw some working codes asfollowinghCx:hopen `:eqkdb8p:12000hCx({select cnt:count i from corrTab};::)I tried to run them in q session and they work very well. The firstline is just to connect to KDB host. But the second line is veryconfusing. Especially corrTab is neither a q keyword nor a variable/table defined. Why it works and print outcnt-------9415683Can anyone help?Thanks

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Date: Fri, 2 Sep 2011 06:15:43 -0400
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For you to understand this you need to read the manuals or q for mortals. In=
particular the sections on ipc, data types (in this case lists) and functio=
ns.

In a nutshell it’s sending a function to the remote host which is simply exe=
cuting the query.

3

hCx ({select cnt:count i from corrTab};::)

is sending the list ({select cnt:count i from corrTab};::) to the socket referred to by hCx.

({select cnt:count i from corrTab};::)

is a 2 element list, a lambda (anonymous function) and identity (::).

It’s possible to evaluate a single level parse tree as

q)value ({x+y};2;3)

and that’s what is happening here, except ({select cnt:count i from corrTab};::) is being recompiled and evaluated remotely.

Presumably corrTab is defined on the remote system.

4

In this case, you are connecting to a different kdb process andsending an anonymous function {select cnt:count i from corrTab} andidentity to be executed in that process. So here corrTab is a tabledefined the q process running on eqkdb8p at port 12000. This isequivalent to executing this query on that remote process “selectcnt:count i from corrTab”. Hope this answers your question.Another way to do the same thing would behCx:hopen :eqkdb8p:12000hCx "select cnt:count i from corrTab"On Sep 2, 5:43?am, Wei Li <lavi...> wrote:&gt; Hi everyone,&gt; Recently I started to learn KDB, and I saw some working codes as&gt; following&gt;&gt; hCx:hopen :eqkdb8p:12000> hCx({select cnt:count i from corrTab};::)>> I tried to run them in q session and they work very well. The first> line is just to connect to KDB host. But the second line is very> confusing. Especially corrTab is neither a q keyword nor a variable/> table defined. Why it works and print out>> cnt> -------> 9415683>> Can anyone help?> Thanks</lavi…>

5

> Another way to do the same thing would be
>
>
> hCx:hopen `:eqkdb8p:12000
> hCx “select cnt:count i from corrTab”

slightly more precisely,

hCx “{select cnt:count i from corrTab}”

sending (::) as an arg is the equivalent of invoking with

–
Aaron Davies
aaron.davies@gmail.com

6

Thanks all for help.Now I understand it.On Sep 3, 1:44?am, Aaron Davies <aaron.dav…> wrote:> > Another way to do the same thing would be>> > hCx:hopen `:eqkdb8p:12000> > hCx “select cnt:count i from corrTab”>> slightly more precisely,>> hCx “{select cnt:count i from corrTab}”>> sending (::) as an arg is the equivalent of invoking with >> –> Aaron Davies> aaron.dav...@gmail.com</aaron.dav…>