The use of presto's projectNode

recently read the presto source code, do not understand the role of projectNode?
only knows that it is used for projection, which can be explained simply.

Hive presto
Mar.20,2021

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MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1e55f59-5a3bf.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1e55f59-5a3bf.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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