You can preface any DQL statement with EXPLAIN to instruct the query engine to return only the execution plan for the statement. This takes the form of a series of operators presented as JSON objects which represent the various component actions the planner has determined as necessary in order to execute the statement.
The statement isn’t executed when using EXPLAIN, only the parsing and planning stages take place.
Example:
DQL
EXPLAIN SELECT * FROM test;
produces: 
{
  "plan": {
    "#operator": "sequence",
    "children": [
      {
        "#operator": "scan",
        "alias": "test",
        "collection": "test",
        "datasource": "default",
        "descriptor": {
          "path": {
            "full_scan": {}
          }
        }
      },
      {
        "#operator": "projection",
        "projections": [
          {
            "expression": "`test`",
            "wildcard": true
          }
        ]
      }
    ]
  }
}
A query plan is read from top to bottom to follow the flow of data through it. In the above example the first action is to scan the collection with a full collection scan (see Access paths) then project the entire document. In the example below, a pair of index scans are intersected for each branch of a union scan and feed a fetch operator before reapplying the filters, grouping, projecting and ordering the results:
DQL
EXPLAIN
SELECT field3,count(*) the_count
FROM test
WHERE (field1 = 1 AND field2 = 2)
OR    (field1 = 2 AND field2 = 1)
GROUP BY field3
ORDER BY the_count DESC

{
  "plan": {
    "#operator": "sequence",
    "children": [
      {
        "#operator": "unionScan",
        "children": [
          {
            "#operator": "intersectScan",
            "children": [
              {
                "#operator": "indexScan",
                "alias": "test",
                "collection": "test",
                "datasource": "default",
                "desc": {
                  "index": "ix1",
                  "spans": [
                    {
                      "index_key": {
                        "direction": "asc",
                        "include_missing": true,
                        "key": [
                          "field1"
                        ]
                      },
                      "range": {
                        "high": {
                          "included": true,
                          "value": 1
                        },
                        "low": {
                          "included": true,
                          "value": 1
                        }
                      }
                    }
                  ]
                }
              },
              {
                "#operator": "indexScan",
                "alias": "test",
                "collection": "test",
                "datasource": "default",
                "desc": {
                  "index": "ix2",
                  "spans": [
                    {
                      "index_key": {
                        "direction": "asc",
                        "include_missing": true,
                        "key": [
                          "field2"
                        ]
                      },
                      "range": {
                        "high": {
                          "included": true,
                          "value": 2
                        },
                        "low": {
                          "included": true,
                          "value": 2
                        }
                      }
                    }
                  ]
                }
              }
            ]
          },
          {
            "#operator": "intersectScan",
            "children": [
              {
                "#operator": "indexScan",
                "alias": "test",
                "collection": "test",
                "datasource": "default",
                "desc": {
                  "index": "ix1",
                  "spans": [
                    {
                      "index_key": {
                        "direction": "asc",
                        "include_missing": true,
                        "key": [
                          "field1"
                        ]
                      },
                      "range": {
                        "high": {
                          "included": true,
                          "value": 2
                        },
                        "low": {
                          "included": true,
                          "value": 2
                        }
                      }
                    }
                  ]
                }
              },
              {
                "#operator": "indexScan",
                "alias": "test",
                "collection": "test",
                "datasource": "default",
                "desc": {
                  "index": "ix2",
                  "spans": [
                    {
                      "index_key": {
                        "direction": "asc",
                        "include_missing": true,
                        "key": [
                          "field2"
                        ]
                      },
                      "range": {
                        "high": {
                          "included": true,
                          "value": 1
                        },
                        "low": {
                          "included": true,
                          "value": 1
                        }
                      }
                    }
                  ]
                }
              }
            ]
          }
        ]
      },
      {
        "#operator": "fetch",
        "alias": "test",
        "collection": "test",
        "datasource": "default"
      },
      {
        "#operator": "filter",
        "condition": "(((`test`.`field1` = 1) AND (`test`.`field2` = 2)) OR ((`test`.`field1` = 2) AND (`test`.`field2` = 1)))"
      },
      {
        "#operator": "groupBy",
        "aggregates": [
          {
            "expr": "true",
            "name": "count(true)"
          }
        ],
        "keys": [
          {
            "alias": "$$(group_by_key_1)$$",
            "expression": "`test`.`field3`"
          }
        ]
      },
      {
        "#operator": "projection",
        "projections": [
          {
            "alias": "field3",
            "expression": "`$$(group_by_key_1)$$`"
          },
          {
            "alias": "the_count",
            "expression": "count(true)"
          }
        ]
      },
      {
        "#operator": "sort",
        "orderBy": [
          {
            "direction": "desc",
            "key": "`$$(projection)$$`.`the_count`"
          }
        ]
      },
      {
        "#operator": "finalProjection"
      }
    ]
  }
}