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Serialize a table as an object

The simplest way to serialize a table is as a single object.

save and load

Keywords save and load let you serialize and write any q object to a file of the same name in the working directory. That includes tables, and is the simplest way to persist one.

q)cities:([]city:`Tokyo`Delhi`Shanghai;pop:37435191 29399141 26317104)

q)key `:.                       / nothing in working directory
`symbol$()
q)save `cities
`:cities
q)key `:.                       / file in working directory
,`cities

q)delete cities from `.         / delete from memory
`.
q)cities
'cities
  [0]  cities
       ^

q)load `cities                  / load from filesystem
`cities
q)cities
city     pop
-----------------
Tokyo    37435191
Delhi    29399141
Shanghai 26317104

Perfect for casual use. For more organized writing and reading we need the keywords used to define save and load.

set and get

Keywords set and get differ from save and load:

  • set is a binary; its left argument says where in the filesystem to write
  • get returns the table value rather than the name of the variable it has been assigned to

Notice the similarity of reading a value from memory to reading it from the filesystem.

q)get `:cities                      / from filesystem
Tokyo   | 37435191
Delhi   | 29399141
Shanghai| 26317104

q)get `cities                       / from memory
city     pop
-----------------
Tokyo    37435191
Delhi    29399141
Shanghai 26317104

q)`:foo/bar/bigcities set cities
`:foo/bar/bigcities
q)get `:foo/bar/bigcities
city     pop
-----------------
Tokyo    37435191
Delhi    29399141
Shanghai 26317104

Enumerations and foreign keys

The city column is a symbol vector, that is, an enumeration. It is represented in memory as indexes into the sym table. Serialization and deserialization survives the session’s sym list.

KDB+ 4.0 2020.10.02 Copyright (C) 1993-2020 Kx Systems
m64/ 12()core 65536MB sjt mackenzie.local 127.0.0.1 EXPIRE ..

q)get `:foo/bar/bigcities
city     pop
-----------------
Tokyo    37435191
Delhi    29399141
Shanghai 26317104

Similarly for foreign keys, enumerated against another table.

q)countries:([country:`China`India`Japan];cont:3#`Asia;code:86 91 81)
q)cities:([]
    city:`Tokyo`Delhi`Shanghai;
    country: `countries$`Japan`India`China;
    pop:37435191 29399141 26317104)
q)`:linked/countries`:linked/cities set'(countries;cities)
`:linked/countries`:linked/cities
q)\\

❯ q
KDB+ 4.0 2020.10.02 Copyright (C) 1993-2020 Kx Systems
m64/ 12()core 65536MB sjt mackenzie.local 127.0.0.1 EXPIRE ..

q)countries:get`:linked/countries
q)cities:get`:linked/cities
q)select city,pop,country.code from cities
city     pop      code
----------------------
Tokyo    37435191 81
Delhi    29399141 91
Shanghai 26317104 86

Use cases

Serialization as an object suits a table that is

  • small relative to memory
  • frequently read
  • has most of its columns required by most queries

Splayed tables

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