adding note regarding sparse/dense arrays

Author: PhilippSalvisberg

docs/4-language-usage/4-control-structures/2-case-if-decode-nvl-nvl2-coalesce/g-4240.md

@@ -7,6 +7,11 @@
 
 The `nvl2` function always evaluates all parameters before deciding which one to use. This can be harmful, if parameter 2 or 3 is either a function call or a select statement, as they will be executed regardless of whether parameter 1 contains a `null` value or not.
 
+Please note that:
+* Varrays are always dense
+* Associative arrays (or index-by tables) are either dense or sparse
+* Nested tables start dense and may become sparse
+
 ## Example (bad)
 
 ``` sql

docs/4-language-usage/4-control-structures/2-case-if-decode-nvl-nvl2-coalesce/g-4250.md

@@ -7,6 +7,11 @@
 
 Conditions are evaluated top to bottom in branches of a `case` statement or chain of `if`/`elsif` statements. The first condition to evaluate as true leads to that branch being executed, the rest will never execute. Having an identical duplicated condition in another branch will never be reached and will be dead code.
 
+Please note that:
+* Varrays are always dense
+* Associative arrays (or index-by tables) are either dense or sparse
+* Nested tables start dense and may become sparse
+
 ## Example (bad)
 
 ``` sql

docs/4-language-usage/4-control-structures/3-flow-control/g-4360.md

@@ -7,6 +7,11 @@
 
 When a loose (also called sparse) array is processed using a *numeric* `for loop` we have to check with all iterations whether the element exist to avoid a `no_data_found` exception. In addition, the number of iterations is not driven by the number of elements in the array but by the number of the lowest/highest element. The more gaps we have, the more superfluous iterations will be done.
 
+Please note that:
+* Varrays are always dense
+* Associative arrays (or index-by tables) are either dense or sparse
+* Nested tables start dense and may become sparse
+
 ## Example (bad)
 
 ``` sql