yaooqinn opened a new pull request #26715: [SPARK-29034][SQL] PostgreSQL 
dialect conformance for string constants with C-style escapes
URL: https://github.com/apache/spark/pull/26715
 
 
   ### What changes were proposed in this pull request?
   
   On one hand, we now use `spark.sql.parser.escapedStringLiterals` to control 
whether to escape string literal or not.
   On the other hand, we use `spark.sql.dialect` to choose spark or PostgreSQL 
dialect. When we use the PostgreSQL dialect, we should obey the C-style escape 
behavior of PostgreSQL.
   
   #### Supported
   > An escape string constant is specified by writing the letter E (upper or 
lower case) just before the opening single quote, e.g., E'foo'.
   #### Not supported
   > When continuing an escape string constant across lines, write E only 
before the first opening quote.
   
   Because PostgreSQL follows the SQL standard that is
   > Two string constants that are only separated by whitespace with at least 
one **newline** are concatenated and effectively treated as if the string had 
been written as one constant.
   , which is hard to follow in Spark's Parser.
   
   ### Why are the changes needed?
   
   PostgreSQL dialect conformance
   
   ### Does this PR introduce any user-facing change?
   <!--
   If yes, please clarify the previous behavior and the change this PR proposes 
- provide the console output, description and/or an example to show the 
behavior difference if possible.
   If no, write 'No'.
   -->
   
   yes, when we use the PostgreSQL dialect, we use 'E' to define an escape 
string constant 
   
   ### How was this patch tested?
   <!--
   If tests were added, say they were added here. Please make sure to add some 
test cases that check the changes thoroughly including negative and positive 
cases if possible.
   If it was tested in a way different from regular unit tests, please clarify 
how you tested step by step, ideally copy and paste-able, so that other 
reviewers can test and check, and descendants can verify in the future.
   If tests were not added, please describe why they were not added and/or why 
it was difficult to add.
   -->
   add ut.

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