Document SQL dialect guidance

docs/source/user-guide/sql/dialect.md

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+
+# SQL Dialect
+
+The included SQL supported in Apache DataFusion mostly follows the [PostgreSQL
+SQL dialect], including:
+
+- The sql parser and [SQL planner]
+- Type checking, analyzer, and type coercions
+- Semantics of functions bundled with DataFusion
+
+Notable exceptions:
+
+- Array/List functions and semantics follow the [DuckDB SQL dialect].
+- DataFusion's type system is based on the [Apache Arrow type system], and the mapping to PostgrSQL types is not always 1:1.
+- DataFusion has its own syntax (dialect) for certain operations (like [`CREATE EXTERNAL TABLE`])
+
+As Apache DataFusion is designed to be fully customizable, systems built on
+DataFusion can and do implement different SQL semantics. Using DataFusion's APs,
+you can provide alternate function definitions, type rules, and/or SQL syntax
+that matches other systems such as Apache Spark or MySQL or your own custom
+semantics.
+
+[postgresql sql dialect]: https://www.postgresql.org/docs/current/sql.html
+[sql planner]: https://docs.rs/datafusion/latest/datafusion/sql/planner/struct.SqlToRel.html
+[duckdb sql dialect]: https://duckdb.org/docs/sql/functions/array
+[apache arrow type system]: https://arrow.apache.org/docs/format/Columnar.html#data-types
+[`create external table`]: ddl.md#create-external-table
+
+## Rationale
+
+SQL Engines have a choice to either use an existing SQL dialect or define their
+own. Using an existing dialect may not fit perfectly as it is hard to match
+semantics exactly (need bug-for-bug compatibility), and is likely not what all
+users want. However, it avoids the (very significant) effort of defining
+semantics as well as documenting and teaching users about them.

docs/source/user-guide/sql/index.rst

@@ -21,6 +21,7 @@ SQL Reference
 .. toctree::
    :maxdepth: 2
 
+   dialect
    data_types
    select
    subqueries