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RDB/IHDB/HDB – What is the best way of a copy of the values of an in-memory table?
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?
Cheers,
Adriano -
3 Replies
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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:
doSomething[`tablename]
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.
Cheers,
Kate
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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
‘type
[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.
Best,
David -
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!
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