Is there a difference in performance between set/save when saving columns to tables?

Question

I have a small utility that checks for new columns for an intraday hdb and adds new columns. At the moment I am using :

.[set;(pth;?[data;();();cls]);{[p;e] .log.error[.z.h;"Failed to save to path [",string[p],"] with error :",e]}[pth;]]

where path is :

`:path_to_hdb/2022.03.31/table01/newDummyThree

and

?[data;();();cls] // just an exec statement 

Would it make any difference to use save instead:

.[save;(pth;?[data;();();cls]);{[p;e] .log.error[.z.h;"Failed to save to path [",string[p],"] with error :",e]}[pth;]]

Answer 1

Yes. If you are adding entire columns to a table then you might want to store it splayed, i.e. as a directory of column files rather than as a single table file. This means using set rather than save.

https://code.kx.com/q/kb/splayed-tables/

But test actual example updates.

Answer 2

As mentioned in the documentation for save:

Use set instead to save

• a variable to a file of a different name
• local data

So set has the advantage of not requiring a global and you can name the file a different name to the name of your in-memory global variable.

There is no difference in how they serialise/write the data. In fact, save uses set under the covers anyway:

q)save
k){$[1=#p:`\:*|`\:x:-1!x;set[x;. *p];   x 0:.h.tx[p 1]@.*p]}'

By the way - you can't use save in the way that you've suggested in your post. save takes a symbol as input and this symbol is the symbol name of your global variable containing the data you want to write.