[GitHub] [spark] dilipbiswal edited a comment on issue #25448: [SPARK-28697][SQL] Invalidate Database/Table names starting with underscore

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dilipbiswal edited a comment on issue #25448: [SPARK-28697][SQL] Invalidate Database/Table names starting with underscore
URL: https://github.com/apache/spark/pull/25448#issuecomment-521533557
 
 
   @cloud-fan @dongjoon-hyun @HyukjinKwon 
   Was just checking the db2 definition of a identifier in [link](https://www.ibm.com/support/knowledgecenter/en/SSEPGG_9.7.0/com.ibm.db2.luw.sql.ref.doc/doc/r0000720.html)
   
   Its defined as following :
   ```
   An ordinary identifier is an uppercase letter followed by zero or more characters, each of which is an uppercase letter, a digit, or the underscore character. Note that lower case letters can be used when specifying an ordinary identifier, but they are converted to uppercase when processed. An ordinary identifier should not be a reserved word.
   ```
   
   Hive seems to have allowed  digit as first character as well.
   
   ```
   Identifier
       :
       (Letter | Digit) (Letter | Digit | '_')*
       | {allowQuotedId()}? QuotedIdentifier  /* though at the language level we allow all Identifiers to be QuotedIdentifiers;
                                                 at the API level only columns are allowed to be of this form */
       | '`' RegexComponent+ '`'
       ;
   ```
   
   Not sure why in spark we allowed "_" as starting char to begin with ? Is it to match some other system ?

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