how to get a max per group and subgroup using jq

Solution 1:

Remove the inner brackets to flatten the array, then group_by both criteria (which makes your criteria an array), and map your max_by onto the result array:

jq 'map(.backup[] + {name}) | group_by([.name, .type]) | map(max_by(.timestamp.stop))'

[
  {
    "timestamp": {
      "start": 1642144383,
      "stop": 1642144386
    },
    "info": {
      "size": 1200934840
    },
    "type": "full",
    "name": "dbname1"
  },
  {
    "timestamp": {
      "start": 1642145388,
      "stop": 1642145392
    },
    "info": {
      "size": 1168586330
    },
    "type": "incr",
    "name": "dbname1"
  },
  {
    "timestamp": {
      "start": 1642144383,
      "stop": 1642144386
    },
    "info": {
      "size": 1200934840
    },
    "type": "full",
    "name": "dbname2"
  },
  {
    "timestamp": {
      "start": 1642144388,
      "stop": 1642144392
    },
    "info": {
      "size": 1168586300
    },
    "type": "incr",
    "name": "dbname2"
  }
]

Demo

Solution 2:

This seems to produce the desired expected output. You need an additional grouping by the .type record, before doing the max_by

map( .backup[] + {name} ) | group_by(.name)[] | 
  group_by(.type) | map(max_by(.timestamp.stop))

jqplay demo