### What changes were proposed in this pull request?
After https://github.com/apache/spark/pull/25158 and https://github.com/apache/spark/pull/25458, SQL features of PostgreSQL are introduced into Spark. AFAIK, both features are implementation-defined behaviors, which are not specified in ANSI SQL.
In such a case, this proposal is to add a configuration `spark.sql.dialect` for choosing a database dialect.
After this PR, Spark supports two database dialects, `Spark` and `PostgreSQL`. With `PostgreSQL` dialect, Spark will:
1. perform integral division with the / operator if both sides are integral types;
2. accept "true", "yes", "1", "false", "no", "0", and unique prefixes as input and trim input for the boolean data type.
### Why are the changes needed?
Unify the external database dialect with one configuration, instead of small flags.
### Does this PR introduce any user-facing change?
A new configuration `spark.sql.dialect` for choosing a database dialect.
### How was this patch tested?
Existing tests.
Closes#25697 from gengliangwang/dialect.
Authored-by: Gengliang Wang <gengliang.wang@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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