Implement prettier SQL unparsing (more human readable)

datafusion-examples/examples/parse_sql_expr.rs

@@ -153,5 +153,14 @@ async fn round_trip_parse_sql_expr_demo() -> Result<()> {
 
     assert_eq!(sql, round_trip_sql);
 
+    // enable pretty-unparsing. This make the output more human-readable
+    // but can be problematic when passed to other SQL engines due to
+    // difference in precedence rules between DataFusion and target engines.
+    let unparser = Unparser::default().with_pretty(true);
+
+    let pretty = "int_col < 5 OR double_col = 8";
+    let pretty_round_trip_sql = unparser.expr_to_sql(&parsed_expr)?.to_string();
+    assert_eq!(pretty, pretty_round_trip_sql);
+
     Ok(())
 }

datafusion-examples/examples/plan_to_sql.rs

@@ -31,9 +31,9 @@ use datafusion_sql::unparser::{plan_to_sql, Unparser};
 /// 1. [`simple_expr_to_sql_demo`]: Create a simple expression [`Exprs`] with
 /// fluent API and convert to sql suitable for passing to another database
 ///
-/// 2. [`simple_expr_to_sql_demo_no_escape`]  Create a simple expression
-/// [`Exprs`] with fluent API and convert to sql without escaping column names
-/// more suitable for displaying to humans.
+/// 2. [`simple_expr_to_pretty_sql_demo`] Create a simple expression
+/// [`Exprs`] with fluent API and convert to sql without extra parentheses,
+/// suitable for displaying to humans
 ///
 /// 3. [`simple_expr_to_sql_demo_escape_mysql_style`]" Create a simple
 /// expression [`Exprs`] with fluent API and convert to sql escaping column
@@ -49,6 +49,7 @@ use datafusion_sql::unparser::{plan_to_sql, Unparser};
 async fn main() -> Result<()> {
     // See how to evaluate expressions
     simple_expr_to_sql_demo()?;
+    simple_expr_to_pretty_sql_demo()?;
     simple_expr_to_sql_demo_escape_mysql_style()?;
     simple_plan_to_sql_demo().await?;
     round_trip_plan_to_sql_demo().await?;
@@ -64,6 +65,17 @@ fn simple_expr_to_sql_demo() -> Result<()> {
     Ok(())
 }
 
+/// DataFusioon can remove parentheses when converting an expression to SQL.
+/// Note that output is intended for humans, not for other SQL engines,
+/// as difference in precedence rules can cause expressions to be parsed differently.
+fn simple_expr_to_pretty_sql_demo() -> Result<()> {
+    let expr = col("a").lt(lit(5)).or(col("a").eq(lit(8)));
+    let unparser = Unparser::default().with_pretty(true);
+    let sql = unparser.expr_to_sql(&expr)?.to_string();
+    assert_eq!(sql, r#"a < 5 OR a = 8"#);
+    Ok(())
+}
+
 /// DataFusion can convert expressions to SQL without escaping column names using
 /// using a custom dialect and an explicit unparser
 fn simple_expr_to_sql_demo_escape_mysql_style() -> Result<()> {

datafusion/expr/src/operator.rs

@@ -218,37 +218,33 @@ impl Operator {
     }
 
     /// Get the operator precedence
-    /// use <https://www.postgresql.org/docs/7.0/operators.htm#AEN2026> as a reference
+    /// use <https://www.postgresql.org/docs/7.2/sql-precedence.html> as a reference
     pub fn precedence(&self) -> u8 {
         match self {
             Operator::Or => 5,
             Operator::And => 10,
-            Operator::NotEq
-            | Operator::Eq
-            | Operator::Lt
-            | Operator::LtEq
-            | Operator::Gt
-            | Operator::GtEq => 20,
-            Operator::Plus | Operator::Minus => 30,
-            Operator::Multiply | Operator::Divide | Operator::Modulo => 40,
+            Operator::Eq | Operator::NotEq | Operator::LtEq | Operator::GtEq => 15,
+            Operator::Lt | Operator::Gt => 20,
+            Operator::LikeMatch
+            | Operator::NotLikeMatch
+            | Operator::ILikeMatch
+            | Operator::NotILikeMatch => 25,
             Operator::IsDistinctFrom
             | Operator::IsNotDistinctFrom
             | Operator::RegexMatch
             | Operator::RegexNotMatch
             | Operator::RegexIMatch
             | Operator::RegexNotIMatch
-            | Operator::LikeMatch
-            | Operator::ILikeMatch
-            | Operator::NotLikeMatch
-            | Operator::NotILikeMatch
             | Operator::BitwiseAnd
             | Operator::BitwiseOr
             | Operator::BitwiseShiftLeft
             | Operator::BitwiseShiftRight
             | Operator::BitwiseXor
             | Operator::StringConcat
             | Operator::AtArrow
-            | Operator::ArrowAt => 0,
+            | Operator::ArrowAt => 30,
+            Operator::Plus | Operator::Minus => 40,
+            Operator::Multiply | Operator::Divide | Operator::Modulo => 45,
         }
     }
 }