Decoding Kdb+'s Mixed List Types: Unveiling the Nuances of .Q.ty and meta

Are you scratching your head over unexpected type classifications in kdb+? You're not alone! This article dives deep into the behavior of .Q.ty and meta when dealing with mixed lists, providing clarity and practical examples to help you avoid common pitfalls. We will clarify how kdb+ determines a column's type, especially when mixed data types are involved.

The Curious Case of Uniform vs. Mixed Lists

kdb+'s .Q.ty function, as documented (https://code.kx.com/q/ref/dotq/#ty-type), distinguishes between uniform lists (uppercase type) and mixed lists. However, real-world scenarios can sometimes present surprising results.

Consider this example:

q).Q.ty(10 10;`abn)
"J"
q).Q.ty 10#(10 10;`abn)
"J"
q)type each(10 10;`abn)
7 -11h
q)type each 10#(10 10;`abn)
7 -11 7 -11 7 -11 7 -11 7 -11h

Here, despite the presence of mixed data types (10 10 which is a Long and `abn which is a symbol), .Q.ty incorrectly reports "J" instead of expected mixed list type.

The meta Function's First-Item Examination

The key to understanding this behavior lies in how the meta function (which utilizes .Q.ty) determines column types. It examines only the first item in each column.

• Impact: If the first item represents a uniform list, meta will report its type accordingly, even if subsequent items introduce mixed types.
• Limitation: This behavior doesn't indicate whether the table can be splayed.

Real-World Scenario: Constructing a table t in kdb+

Let's illustrate this with an example table.

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

Even though column f contains mixed types, the meta function identifies its type as "J" (long) because the first element (10 10) is a list of longs.

Forcing Meta to recognize Mixed List Type

To force meta to recognize a mixed list type, ensure that the first row of the column contains a mixed-type element:

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)
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|

With the first element of column f explicitly defined as a mixed list (10;10;abn), metanow correctly identifiesfas amixed list` type.

Diving into .Q.ty and Meta

• .Q.ty Definition: k){$[0h>x:tx x;.q.upper t@-x;t x]}
• meta Definition: k){([!c].Q.ty't;f:.Q.fk't;a:-2!'t:. c:.Q.V x)}

As revealed by Terry Lynch, meta calls .Q.ty, which in turn utilizes .Q.tx. Crucially, .Q.tx employs $[0<t:@*x;-t;0h] which effectively translates to "type first x".

Key Takeaways for Working with Mixed Lists in kdb+

• meta examines only the first item to determine the type of each column.
• To get accurate type reporting for mixed lists, ensure the first item in a column reflects the mixed nature.
• Be aware of this behavior when designing tables and interpreting meta output.
• Consider using type each to inspect the types of individual elements within a column to gain a detailed understanding.

By understanding these nuances, you can effectively work with kdb+ mixed list types and leverage the power of meta to gain accurate insights into your data structures.