Why is it that when String type asks for SUM, the query speed with round, is very fast, and the query speed without round, is very slow?

Why is it that when executing a SUM query in the code, the query with round (), is fast, the query without round (), is slow, and the execution speed is fast in Navicat?
1, xml file, add round ()
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sql:
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2xmlround

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sql:

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Feb.21,2022

explain sql. What's the difference between taking a look at the execution plan?

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-64f5-1b9c72f-2f324.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b9c72f-2f324.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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