array.diff

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File	+	−
doc/src/sgml/array.sgml	31	1
src/backend/utils/adt/arrayfuncs.c	6	0

diff --git a/doc/src/sgml/array.sgml b/doc/src/sgml/array.sgml
index bb4657e..dc7b6f4 100644
--- a/doc/src/sgml/array.sgml
+++ b/doc/src/sgml/array.sgml
@@ -224,7 +224,9 @@ SELECT name FROM sal_emp WHERE pay_by_quarter[1] <> pay_by_quarter[2];
   By default <productname>PostgreSQL</productname> uses a
   one-based numbering convention for arrays, that is,
   an array of <replaceable>n</> elements starts with <literal>array[1]</literal> and
-  ends with <literal>array[<replaceable>n</>]</literal>.
+  ends with <literal>array[<replaceable>n</>]</literal>. Negative
+  array subscript numbers indicate that the position of the element is calculated from
+  the end of the array, with -1 indicating the last element in the array.
  </para>
 
  <para>
@@ -242,6 +244,34 @@ SELECT pay_by_quarter[3] FROM sal_emp;
  </para>
 
  <para>
+  This query retrieves the last quarter pay of all employees:
+
+<programlisting>
+SELECT pay_by_quarter[-1] FROM sal_emp;
+
+ pay_by_quarter
+----------------
+          10000
+          25000
+(2 rows)
+</programlisting>
+</para>
+
+<para>
+  This query retrieves the pay of all employees for the last three quarters:
+<programlisting>
+SELECT pay_by_quarter[-3:4] FROM sal_emp;
+
+   pay_by_quarter
+---------------------
+ {10000,10000,10000}
+ {25000,25000,25000}
+(2 rows)
+
+</programlisting>
+ </para>
+
+ <para>
   We can also access arbitrary rectangular slices of an array, or
   subarrays.  An array slice is denoted by writing
   <literal><replaceable>lower-bound</replaceable>:<replaceable>upper-bound</replaceable></literal>
diff --git a/src/backend/utils/adt/arrayfuncs.c b/src/backend/utils/adt/arrayfuncs.c
index fb4cbce..9d6c3f1 100644
--- a/src/backend/utils/adt/arrayfuncs.c
+++ b/src/backend/utils/adt/arrayfuncs.c
@@ -1786,6 +1786,8 @@ array_ref(ArrayType *array,
 	}
 	for (i = 0; i < ndim; i++)
 	{
+	   	if (indx[i] < 0) /* A negative index number indicates a position calculated from the end of the array */
+	           	indx[i] = dim[i] + indx[i] + lb[i];
 		if (indx[i] < lb[i] || indx[i] >= (dim[i] + lb[i]))
 		{
 			*isNull = true;
@@ -1914,6 +1916,10 @@ array_get_slice(ArrayType *array,
 
 	for (i = 0; i < nSubscripts; i++)
 	{
+	   	if (lowerIndx[i] < 0) /* A negative index number indicates a position calculated from the end of the array */
+	           	lowerIndx[i] = dim[i] + lowerIndx[i] + lb[i];
+	   	if (upperIndx[i] < 0) /* A negative index number indicates a position calculated from the end of the array */
+	           	upperIndx[i] = dim[i] + upperIndx[i] + lb[i];
 		if (lowerIndx[i] < lb[i])
 			lowerIndx[i] = lb[i];
 		if (upperIndx[i] >= (dim[i] + lb[i]))