wangyum opened a new pull request #24551: [SPARK-27655][SQL] Persistent the 
table statistics to metadata after fall back to hdfs
URL: https://github.com/apache/spark/pull/24551
 
 
   ## What changes were proposed in this pull request?
   
   It's a real case. We need to join many times from different tables, but the 
statistics of some tables are incorrect. So this job need 43 min. It only need 
3.7 min after `set spark.sql.statistics.persistentStatsAfterFallBack=true`. But 
the newly calculated statistics just for this SparkSession. So we need to 
recalculate statistics in the next time we start a new SparkSession.
   This PR adds a switch to start a new thread to persistent statistics to the 
Hive metastore.
   
   This is our job:
   
   -  default(spark.sql.statistics.persistentStatsAfterFallBack=false):
   
![image](https://issues.apache.org/jira/secure/attachment/12968145/disableFallBackToHdfs.png)
   -  spark.sql.statistics.persistentStatsAfterFallBack=true:
   
![image](https://issues.apache.org/jira/secure/attachment/12968144/enableFallBackToHdfs.png)
   ## How was this patch tested?
   
   unit tests and manual tests

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