From 350d4ca8a895a53f51935741ee35e5b79120829c Mon Sep 17 00:00:00 2001
From: Mustafa Akur <106137913+mustafasrepo@users.noreply.github.com>
Date: Thu, 27 Jul 2023 23:31:49 +0300
Subject: [PATCH] Update document. (#7111)

---
 datafusion/sql/src/select.rs | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/datafusion/sql/src/select.rs b/datafusion/sql/src/select.rs
index daf79e969e1f..59656587fa79 100644
--- a/datafusion/sql/src/select.rs
+++ b/datafusion/sql/src/select.rs
@@ -562,7 +562,28 @@ fn match_window_definitions(
     Ok(())
 }
 
-/// Update group by exprs, according to functioanl dependencies
+/// Update group by exprs, according to functional dependencies
+/// The query below
+///
+/// SELECT sn, amount
+/// FROM sales_global
+/// GROUP BY sn
+///
+/// cannot be calculated, because it has a column(`amount`) which is not
+/// part of group by expression.
+/// However, if we know that, `sn` is determinant of `amount`. We can
+/// safely, determine value of `amount` for each distinct `sn`. For these cases
+/// we rewrite the query above as
+///
+/// SELECT sn, amount
+/// FROM sales_global
+/// GROUP BY sn, amount
+///
+/// Both queries, are functionally same. \[Because, (`sn`, `amount`) and (`sn`)
+/// defines the identical groups. \]
+/// This function updates group by expressions such that select expressions that are
+/// not in group by expression, are added to the group by expressions if they are dependent
+/// of the sub-set of group by expressions.
 fn get_updated_group_by_exprs(
     group_by_exprs: &[Expr],
     select_exprs: &[Expr],