sub-categorize special function forms

this is the tutorial, which should have some semblence of
not getting too far into the weeds.  however, as we dont
really have other places to explain SQL concepts, and SQL
functions have a lot of them, we dont have another home right
now.  so at least further sub-categorize window functions,
table/column valued functions, and WITHIN GROUP into an
"advanced function techniques" section with a disclaimer that
these are less common use cases.

Change-Id: I4b16bd5673c0bd39a9b880338f6ce3cfbafbe271

diff --git a/doc/build/tutorial/data_select.rst b/doc/build/tutorial/data_select.rst
index f30f7a587369a6581db341a5956b4450cdd95e7d..6d52b4627324508d27f22d78572453cfe67920af 100644 (file)
--- a/doc/build/tutorial/data_select.rst
+++ b/doc/build/tutorial/data_select.rst
@@ -1534,10 +1534,19 @@ Overall, the scenario where the
    or again special datatypes such as :class:`_types.JSON`,
    :class:`_types.ARRAY`.
 
+Advanced SQL Function Techniques
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The following subsections illustrate more things that can be done with
+SQL functions.  While these techniques are less common and more advanced than
+basic SQL function use, they nonetheless are extremely popular, largely
+as a result of PostgreSQL's emphasis on more complex function forms, including
+table- and column-valued forms that are popular with JSON data.
+
 .. _tutorial_window_functions:
 
 Using Window Functions
-~~~~~~~~~~~~~~~~~~~~~~
+######################
 
 A window function is a special use of a SQL aggregate function which calculates
 the aggregate value over the rows being returned in a group as the individual
@@ -1613,7 +1622,7 @@ Further options for window functions include usage of ranges; see
 .. _tutorial_functions_within_group:
 
 Special Modifiers WITHIN GROUP, FILTER
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+######################################
 
 The "WITHIN GROUP" SQL syntax is used in conjunction with an "ordered set"
 or a "hypothetical set" aggregate
@@ -1653,7 +1662,7 @@ using the :meth:`_functions.FunctionElement.filter` method::
 .. _tutorial_functions_table_valued:
 
 Table-Valued Functions
-~~~~~~~~~~~~~~~~~~~~~~~~~
+#######################
 
 Table-valued SQL functions support a scalar representation that contains named
 sub-elements. Often used for JSON and ARRAY-oriented functions as well as
@@ -1708,7 +1717,7 @@ towards as ``value``, and then selected two of its three rows.
 .. _tutorial_functions_column_valued:
 
 Column Valued Functions - Table Valued Function as a Scalar Column
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+##################################################################
 
 A special syntax supported by PostgreSQL and Oracle is that of referring
 towards a function in the FROM clause, which then delivers itself as a