CASE WHEN vs UNION SELECT TEST

CASE_WHEN_EXPLAIN

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            "covers": [
              "cover ((`default`.`fieldX`))",
              "cover ((`default`.`fieldY`))",
              "cover ((`default`.`fieldA`))",
              "cover ((`default`.`fieldB`))",
              "cover ((`default`.`fieldC`))",
              "cover ((`default`.`fieldD`))",
              "cover ((`default`.`fieldE`))",
              "cover ((`default`.`fieldF`))",
              "cover ((`default`.`fieldG`))",
              "cover ((`default`.`fieldH`))",
              "cover ((`default`.`field1`))",
              "cover ((`default`.`field2`))",
              "cover ((`default`.`field3`))",
              "cover ((`default`.`field4`))",
              "cover ((meta(`default`).`id`))"
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            "index": "idx_case_when_union_test_1",
            "index_id": "c8f6007b8337e30f",
            "index_projection": {
              "entry_keys": [
                0,
                1
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            "keyspace": "default",
            "namespace": "default",
            "spans": [
              {
                "exact": true,
                "range": [
                  {
                    "high": "\"X\"",
                    "inclusion": 3,
                    "low": "\"X\""
                  },
                  {
                    "high": "\"Y\"",
                    "inclusion": 3,
                    "low": "\"Y\""
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