kdb - How to pass a table by reference to kdb function - kdb

Question

Define the question
Given an empty table myt defined by
myt:([] id:`int$(); score:`int$())
It is trivial to insert one or more records into it, for example
`myt upsert `id`score!1 100
But when it comes to defining a function to insert into a given table, it seems a different trick.
A first try version could be
upd:{[t] t upsert `id`score!42 314;}
upd[myt]
Apparently it updates nothing to myt itself but a local copy version of it.
Difficulties of Possible solutions
Possible solution 1: using the global variable instead
Let myt be a global variable, the variable will then be accessed inside a function.
upd:{`myt upsert `id`score!42 314;}
upd[]
It looks a good solution, expect if many myts are required. Under this situation, one have to provide a lot of copy for upd function as following
upd0:{`myt0 upsert `id`score!42 314;}
upd1:{`myt1 upsert `id`score!42 314;}
upd2:{`myt2 upsert `id`score!42 314;}
...
So, the global variable solution is not a good solution here.
Possible solution 2: amending table outside function
One can also solve the problem by amending myt just outside the function, returning the modified result by removing the ending ;.
upd:{[t] t upsert `id`score!42 314} / return inserted valued
myt:upd[myt]
It works! But after running this code for millions of times, it works slower and slower. Because this solution discards the "in-place" property of upsert operator, the copy overhead increases as the size of table getting larger.
Pass argument by reference?
Maybe the concept of "pass-by-reference" solution here. Or maybe q has its own solution for this problem and I have not get the essential idea.
[UPDATE] Solved by adding "`" to call-by-name
As cillianreilly answers, it is simple to add a "`" symbol in front of myt to declare it as a global variable when pass it into function. So the perfect solution is direct.
upd:{[t] t upsert `id`score!42 314;}
upd[`myt] / it works

Answer 1

Your first version should achieve what you want. If you pass the table name as a symbol, it will update the global variable and return the table name. If you pass the table itself, it will return the updated table, which you can use in an assignment, as you found in possible solution 2. Note that the actual table will not have been updated by this operation.
q){[t;x]t upsert x}[myt;`id`score!42 314]
id score
--------
42 314
q)count myt
0
q){[t;x]t upsert x}[`myt;`id`score!42 314]
`myt
q)count myt
1
For possible solution 1, why would you need hundreds of myt tables? Regardless, there is no need to hardcode the table name into the function. You can just pass the table name as a symbol as demonstrated above, which will update the global for you. The official kx kdb tick example given on their github uses insert for exactly this scenario, but in practice a lot of developers use upsert. https://github.com/KxSystems/kdb-tick/blob/master/tick/r.q#L6
Hope this helps.