CODEWITHSUNDEEP
jsonjq
EJT
EJT

Reputation: 45

Using jq to extract common prefixes in a JSON data structure

I have a JSON data set with around 8.7 million key value pairs extracted from a Redis store, where each key is guaranteed to be an 8 digit number, and the key is an 8 alphanumeric character value i.e.

[{
"91201544":"INXX0019",
"90429396":"THXX0020",
"20140367":"ITXX0043",
 ...
}]

To reduce Redis memory usage, I want to transform this into a hash of hashes, where the hash prefix key is the first 6 characters of the key (see this link) and then store this back into Redis.

Specifically, I want my resulting JSON data structure (that I'll then write some code to parse this JSON structure and create a Redis command file consisting of HSET, etc) to look more like

[{
 "000000": { "00000023": "INCD1234",
             "00000027": "INCF1423",
              ....
           },
 ....
 "904293": { "90429300": "THXX0020",
             "90429302": "THXX0024",
             "90429305": "THXY0013"}
 }]

Since I've been impressed by jq and I'm trying to be more proficient at functional style programming, I wanted to use jq for this task. So far I've come up with the following:

% jq '.[0] | to_entries | map({key: .key, pfx: .key[0:6], value: .value}) | group_by(.pfx)'

This gives me something like

[
  [
    {
      "key": "00000130",
      "pfx": "000001",
      "value": "CAXX3231"
    },
    {
      "key": "00000162",
      "pfx": "000001",
      "value": "CAXX4606"
    }
  ],
  [
    {
      "key": "00000238",
      "pfx": "000002",
      "value": "CAXX1967"
    },
    {
      "key": "00000256",
      "pfx": "000002",
      "value": "CAXX0727"
    }
  ],
  ....
]

I've tried the following:

% jq 'map(map({key: .pfx, value: {key, value}})) 
      | map(reduce .[] as $item ({}; {key: $item.key, value: [.value[], $item.value]} )) 
      | map( {key, value: .value | from_entries} ) 
      | from_entries'

which does give me the correct result, but also prints out an error for every reduce (I believe) of 

jq: error: Cannot iterate over null

The end result is 

{
   "000001": {
     "00000130": "CAXX3231",
     "00000162": "CAXX4606"
   },
   "000002": {
     "00000238": "CAXX1967",
     "00000256": "CAXX0727"
   },
   ...
}

which is correct, but how can I avoid getting this stderr warning thrown as well?

Upvotes: 3

Views: 3297

Answers (3)

peak
peak

Reputation: 116750

For the record, jq's group_by relies on sorting, which of course will slow things down noticeably when the input is sufficiently large. The following is about 40% faster even when the input array has just 100,000 items:

def compress:
  . as $in
  | reduce keys[] as $key ({};
      $key[0:6] as $k6
      | $key[6:] as $k2
      | .[$k6] += {($k2): $in[$key]} );

.[0] | compress

Given Jeff's input, the output is identical.

Upvotes: 0

max taldykin
max taldykin

Reputation: 12898

I understand that this is not what you are asking for but, just for the reference, I think it will be MUCH more faster to do this with Redis's built-in Lua scripting.

And it turns out that it is a bit more straightforward:

for _,key in pairs(redis.call('keys', '*')) do
  local val = redis.call('get', key)
  local short_key = string.sub(key, 0, -2)
  redis.call('hset', short_key, key, val)
  redis.call('del', key)
end

This will be done in place without transferring from/to Redis and converting to/from JSON.

Run it from console as:

$ redis-cli eval "$(cat script.lua)" 0

Upvotes: 0

Jeff Mercado
Jeff Mercado

Reputation: 134881

I'm not sure there's enough data here to assess what the source of the problem is. I find it hard to believe that what you tried results in that. I'm getting errors with that all the way.

Try this filter instead:

.[0]
    | to_entries
    | group_by(.key[0:6])
    | map({
          key:   .[0].key[0:6],
          value: map(.key=.key[6:8]) | from_entries
      })
    | from_entries

Given data that looks like this:

[{
    "91201544":"INXX0019",
    "90429396":"THXX0020",
    "20140367":"ITXX0043",
    "00000023":"INCD1234",
    "00000027":"INCF1423",
    "90429300":"THXX0020",
    "90429302":"THXX0024",
    "90429305":"THXY0013"
}]

Results in this:

{
  "000000": {
    "23": "INCD1234",
    "27": "INCF1423"
  },
  "201403": {
    "67": "ITXX0043"
  },
  "904293": {
    "00": "THXX0020",
    "02": "THXX0024",
    "05": "THXY0013",
    "96": "THXX0020"
  },
  "912015": {
    "44": "INXX0019"
  }
}

Upvotes: 2

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