Decoding Kdb+ Mixed Lists: Unveiling the Mysteries of .Q.ty and meta

Are you scratching your head over unexpected type behavior in Kdb+ when dealing with lists? You're not alone! Many developers encounter situations where meta and .Q.ty don't seem to reflect the true nature of their data. This deep dive unravels the intricacies of identifying kdb+ mixed list type and provides clarity on how meta and .Q.tyfunction.

The Curious Case of Uniform List Types

Kdb+ documentation states that .Q.ty returns uppercase letters to signify a list of uniform type.

q).Q.ty(10 10;`abn)
"J"
q).Q.ty 10#(10 10;`abn)
"J"

But what happens when you have lists containing different data types? Let's investigate.

Understanding .Q.ty and Its Limitations

The .Q.ty function, used internally by meta, determines the type of a list. However, it only examines the first item in the list to make this determination.

q)type each(10 10;`abn)
7 -11h

This behavior can lead to confusion when dealing with kdb+ mixed type list, where subsequent items might have different types than the first.

Meta's Perspective: A First Impression

The meta command provides metadata about a table, including the data types of each column. But, like .Q.ty, meta only considers the first entry in each column.

q)show t:([]a:3 4 5;b:"abc";c:(3;"xy";`ab);d:3 2#3 4 5;e:("abc";"de";"fg");f:(10 10;`abn;10 10))
a b c d e f
------------------------
3 a 3 3 4 "abc" 10 10
4 b "xy" 5 3 "de" `abn
5 c `ab 4 5 "fg" 10 10
q)meta t
c| t f a
-| -----
a| j
b| c
c|
d| J
e| C
f| J

Notice that column f, which appears to contain a kdb+ mixed list type, is reported as type J (long) because the first element is a list of longs.

Demonstrating True Mixed Type

To force meta to recognize a column as a truly mixed type, the first element must be a mixed list:

q)show t:([]a:3 4 5;b:"abc";c:(3;"xy";`ab);d:3 2#3 4 5;e:("abc";"de";"fg");f:((10;10;`abn); 10; 10 10))
a b c d e f
-------------------------------
3 a 3 3 4 "abc" (10;10;`abn)  //The first element in column f is a mixed list
4 b "xy" 5 3 "de" 10
5 c `ab 4 5 "fg" 10 10
q)meta t
c| t f a
-| -----
a| j
b| c
c|
d| J
e| C
f|

Now, meta correctly indicates that column f has a mixed type (empty string).

Key Takeaways for Working with Mixed Lists in Kdb+

• .Q.ty and meta are superficial: They only examine the first element of a list or column.
• Force mixed type recognition: Ensure the first element of a potential mixed-type column is, in fact, a mixed list.
• Be explicit: When designing your data structures, consider the implications of mixed types and whether they truly represent your data.
• Consider alternatives: If performance is critical, explore alternative data structures that avoid mixed types if possible.

By understanding the nuances of meta and .Q.ty, you can avoid common pitfalls and gain a more accurate picture of your data types in Kdb+.