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Q Language - Attributes
Lists, dictionaries, or columns of a table can have attributes applied to them. Attributes impose certain properties on the list. Some attributes might disappear on modification.
Types of Attributes
`s# means the list is sorted in an ascending order. If a list is explicitly sorted by asc (or xasc), the list will automatically have the sorted attribute set.
q)L1: asc 40 30 20 50 9 4 q)L1 `s#4 9 20 30 40 50
A list which is known to be sorted can also have the attribute explicitly set. Q will check if the list is sorted, and if is not, an s-fail error will be thrown.
q)L2:30 40 24 30 2 q)`s#L2 's-fail
The sorted attribute will be lost upon an unsorted append.
`p# means the list is parted and identical items are stored contiguously.
The range is an int or temporal type having an underlying int value, such as years, months, days, etc. You can also partition over a symbol provided it is enumerated.
Applying the parted attribute creates an index dictionary that maps each unique output value to the position of its first occurrence. When a list is parted, lookup is much faster, since linear search is replaced by hashtable lookup.
q)L:`p# 99 88 77 1 2 3 q)L `p#99 88 77 1 2 3 q)L,:3 q)L 99 88 77 1 2 3 3
The parted attribute is not preserved under an operation on the list, even if the operation preserves the partitioning.
The parted attribute should be considered when the number of entities reaches a billion and most of the partitions are of substantial size, i.e., there is significant repetition.
`g# means the list is grouped. An internal dictionary is built and maintained which maps each unique item to each of its indices, requiring considerable storage space. For a list of length L containing u unique items of size s, this will be (L × 4) + (u × s) bytes.
Grouping can be applied to a list when no other assumptions about its structure can be made.
The attribute can be applied to any typed lists. It is maintained on appends, but lost on deletes.
q)L: `g# 1 2 3 4 5 4 2 3 1 4 5 6 q)L `g#1 2 3 4 5 4 2 3 1 4 5 6 q)L,:9 q)L `g#1 2 3 4 5 4 2 3 1 4 5 6 9 q)L _:2 q)L 1 2 4 5 4 2 3 1 4 5 6 9
Applying the unique attribute (`u#) to a list indicates that the items of the list are distinct. Knowing that the elements of a list are unique dramatically speeds up distinct and allows q to execute some comparisons early.
When a list is flagged as unique, an internal hash map is created to each item in the list. Operations on the list must preserve uniqueness or the attribute is lost.
q)LU:`u#`MSFT`SAMSUNG`APPLE q)LU `u#`MSFT`SAMSUNG`APPLE q)LU,:`IBM /Uniqueness preserved q)LU `u#`MSFT`SAMSUNG`APPLE`IBM q)LU,:`SAMSUNG / Attribute lost q)LU `MSFT`SAMSUNG`APPLE`IBM`SAMSUNG
`u# is preserved on concatenations which preserve the uniqueness. It is lost on deletions and non-unique concatenations.
Searches on `u# lists are done via a hash function.
Attributes can be removed by applying `#.
Three formats for applying attributes are −
L: `s# 14 2 3 3 9/ Specify during list creation
@[ `.; `L ; `s#]/ Functional apply, i.e. to the variable list L
/ in the default namespace (i.e. `.) apply
/ the sorted `s# attribute
Update `s#time from `tab
/ Update the table (tab) to apply the
Let’s apply the above three different formats with examples.
q)/ set the attribute during creation q)L:`s# 3 4 9 10 23 84 90 q)/apply the attribute to existing list data q)L1: 9 18 27 36 42 54 q)@[`.;`L1;`s#] `. q)L1 / check `s#9 18 27 36 42 54 q)@[`.;`L1;`#] / clear attribute `. q)L1 9 18 27 36 42 54 q)/update a table to apply the attribute q)t: ( sym:`ibm`msft`samsung; mcap:9000 18000 27000) q)t:(time:09:00 09:30 10:00t;sym:`ibm`msft`samsung; mcap:9000 18000 27000) q)t time sym mcap --------------------------------- 09:00:00.000 ibm 9000 09:30:00.000 msft 18000 10:00:00.000 samsung 27000 q)update `s#time from `t `t q)meta t / check it was applied c | t f a ------ | ----- time | t s sym | s mcap | j Above we can see that the attribute column in meta table results shows the time column is sorted (`s#).