{
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    {
      "join_preparation": {
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        "steps": [
          {
            "expanded_query": "/* select#1 */ select count(distinct `tt`.`a`) AS `count(distinct a)` from `tt`"
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        ]
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    {
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              {
                "table": "`tt`",
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                "depends_on_map_bits": [
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          {
            "rows_estimation": [
              {
                "table": "`tt`",
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                  "keys": [
                    "PRIMARY",
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                      "key_parts": [
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                        "cause": "primary_key_is_clustered"
                      },
                      {
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                    ]
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                }
              }
            ]
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          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
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                "table": "`tt`",
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          {
            "attaching_conditions_to_tables": {
              "original_condition": null,
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              ],
              "attached_conditions_summary": [
                {
                  "table": "`tt`",
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              ]
            }
          },
          {
            "refine_plan": [
              {
                "table": "`tt`"
              }
            ]
          },
          {
            "creating_tmp_table": {
              "tmp_table_info": {
                "table": "intermediate_tmp_table",
                "row_length": 5,
                "key_length": 4,
                "unique_constraint": false,
                "location": "memory (heap)",
                "row_limit_estimate": 3355443
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            }
          }
        ]
      }
    },
    {
      "join_execution": {
        "select#": 1,
        "steps": [
        ]
      }
    }
  ]
}