title: what are "ReverseTransform" and "MergeTreeReverse" mean in the clickhouse explain pipeline?
body: <p>what are &quot;ReverseTransform&quot; and  &quot;MergeTreeReverse&quot; mean in the clickhouse explain pipeline?</p>
  <p>table</p>
  <pre class="lang-sql prettyprint-override"><code>CREATE TABLE device_info
  (
  `event_time` DateTime,
  `create_time` DateTime DEFAULT now(),
  `product_id` String,
  `platform` String,
  `app_version` String,
  `sdk_version` String,
  `os_version` Strin,
  `model` String,
  `device_id` String,
  `device_type` String,
  `device_cost` Int64,
  `data_id` String
  )
  ENGINE = ReplicatedMergeTree()
  PARTITION BY toYYYYMMDD(event_time)
  ORDER BY (event_time, product_id, platform, app_version, sdk_version, os_version, model)
  TTL event_time + toIntervalDay(7)
  SETTINGS index_granularity = 8192 
  </code></pre>
  <pre class="lang-sql prettyprint-override"><code>SELECT
     event_time AS eventTime,
     launch_cost AS launchCost,
     device_id,
     data_id
  FROM device_info
  WHERE (device_id = 'xxxxxxx') AND (product_id = 'xxxxxxx') AND (device_type IN ('type1')) AND (event_time &gt;= 'xxxx') AND (event_time &lt;= 'xxxxx')
  ORDER BY event_time DESC
  </code></pre>
  <p>explain pipeline</p>
  <p>what are &quot;ReverseTransform&quot; and  &quot;MergeTreeReverse&quot; mean in the explain pipeline?</p>
  <p><a href="https://i.stack.imgur.com/svgGG.png" rel="nofollow noreferrer">explain pipeline</a></p>
  <p>in the trace log I find a statement is
  &quot;MergingSortedTransform: Merge sorted 645 blocks,861 rows in 5.927999977 sec. what is the  statement mean?</p>
  <p>appreciate for your answer! thx</p>