Set specified columns as primary keys of a table

x xkey y    xkey[x;y]

Where symbol atom or vector x lists columns in table y, which is passed by

• value, returns
• reference, updates

y with x set as the primary keys.

q)\l trade.q
q)keys trade
`symbol$()            / no primary key
q)`sym xkey trade     / return table with primary key sym
sym| time         price size
---| -----------------------
a  | 09:30:00.000 10.75 100
q)keys trade         / trade has not changed
`symbol$()
q)`sym xkey `trade   / pass trade by reference updates the table in place
`trade
q)keys trade         / sym is now primary key of trade
,`sym