From a788d0b124d9f20bc0e05e8a825920925f371460 Mon Sep 17 00:00:00 2001
From: ivan-klass <ivan.klass@performi.ca>
Date: Thu, 4 May 2023 13:03:27 +0200
Subject: [PATCH] Note for multiple arrayJoin of same array expression

---
 docs/en/sql-reference/functions/array-join.md | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/docs/en/sql-reference/functions/array-join.md b/docs/en/sql-reference/functions/array-join.md
index 14968eb1092..98675f02f43 100644
--- a/docs/en/sql-reference/functions/array-join.md
+++ b/docs/en/sql-reference/functions/array-join.md
@@ -78,6 +78,22 @@ GROUP BY
 │           1 │ Bobruisk │ Firefox │
 └─────────────┴──────────┴─────────┘
 ```
+### Important note!
+Using multiple `arrayJoin` with same expression may not produce expected results due optimizations.
+For that cases, consider changing array expression that is repeated with extra operations that do not affect join result - e.g. `arrayJoin(arraySort(arr))`, `arrayJoin(arrayConcat(arr, []))`
+
+Example:
+```sql
+SELECT
+    arrayJoin(dice) as first_throw,
+    /* arrayJoin(dice) as second_throw */ -- is technically correct, but will annihilate result set
+    arrayJoin(arrayConcat(dice, [])) as second_throw -- intentionally changed expression to force re-evaluation
+FROM (
+    SELECT [1, 2, 3, 4, 5, 6] as dice
+);
+```
+
+
 
 Note the [ARRAY JOIN](../statements/select/array-join.md) syntax in the SELECT query, which provides broader possibilities.
 `ARRAY JOIN` allows you to convert multiple arrays with the same number of elements at a time.