For Each Attribute, When Is It Preserved and When Is It Lost?

Question

Describe when each of the four attributes is kept, and when it is lost, as an in-memory list is appended to, has items dropped, or has items modified.

Solution

/ SORTED (s#)
n: `s# 1 2 3 5 8
n,: 13          / kept — 13 is not less than the last item
attr n          / `s
n,: 1           / lost — 1 breaks the ascending order
attr n          / `

/ UNIQUE (u#)
u: `u# `AAPL`GOOG`MSFT
u,: `TSLA       / kept — TSLA is new
attr u          / `u
attr -1_u       / ` — dropping items removes it
u,: `AAPL       / no error, but AAPL is now a duplicate...
attr u          / ` — ...so the attribute is removed

/ PARTED (p#)
p: `p# `A`A`B`B`C
p,: `C          / even this append, which keeps the blocks contiguous...
attr p          / ` — ...removes p#

/ GROUPED (g#)
g: `g# `A`B`A`C
g,: `B`D        / kept on append
attr g          / `g
g[0]: `Z        / kept when items are replaced
attr g          / `g
attr -1_g       / ` — dropping items removes it

Explanation

Sorted `s#: kept on an in-place append (,:) when the new items keep the list in ascending order; lost otherwise. Lost when items are dropped or modified.

Unique `u#: kept on append when the new items are still unique. Appending a duplicate does not signal an error — the append succeeds and the attribute is silently removed. Lost when items are dropped or modified.

Parted `p#: KX's documentation states that "the parted attribute is removed by any operation on the list". In memory, that includes every append, even one that would keep equal values contiguous.

Grouped `g#: kept on append, whatever the new values are, and kept when items are replaced — kdb+ updates its index. Lost when items are dropped (for example with _, or delete on a table).

Summary table:

Attribute	Append that keeps the property	Append that breaks it	Drop items	Modify an item
`s#	Kept	Lost	Lost	Lost
`u#	Kept	Lost (no error)	Lost	Lost
`p#	Lost	Lost	Lost	Lost
`g#	Kept	— (any value is valid)	Lost	Kept

Note also that a plain join (n , x) always returns a new list without an attribute — use the in-place form n ,: x if you want the rules above to apply.

On disk, KX notes that only s# is kept when a column file is appended to. This is why p# must be re-applied after appending to an HDB partition, and why g# — kept through appends in memory — is the attribute used on the RDB's sym column.