Repair corner-case bug in array version of percentile_cont().

The code for advancing through the input rows overlooked the case that we
might already be past the first row of the row pair now being considered,
in case the previous percentile also fell between the same two input rows.

Report and patch by Andrew Gierth; logic rewritten a bit for clarity by me.

src/backend/utils/adt/orderedsetaggs.c

@@ -904,42 +904,50 @@ percentile_cont_multi_final_common(FunctionCallInfo fcinfo,
 
 		for (; i < num_percentiles; i++)
 		{
-			int64		target_row = pct_info[i].first_row;
-			bool		need_lerp = (pct_info[i].second_row > target_row);
+			int64		first_row = pct_info[i].first_row;
+			int64		second_row = pct_info[i].second_row;
 			int			idx = pct_info[i].idx;
 
-			/* Advance to first_row, if not already there */
-			if (target_row > rownum)
+			/*
+			 * Advance to first_row, if not already there.  Note that we might
+			 * already have rownum beyond first_row, in which case first_val
+			 * is already correct.  (This occurs when interpolating between
+			 * the same two input rows as for the previous percentile.)
+			 */
+			if (first_row > rownum)
 			{
-				if (!tuplesort_skiptuples(osastate->sortstate, target_row - rownum - 1, true))
+				if (!tuplesort_skiptuples(osastate->sortstate, first_row - rownum - 1, true))
 					elog(ERROR, "missing row in percentile_cont");
 
 				if (!tuplesort_getdatum(osastate->sortstate, true, &first_val, &isnull) || isnull)
 					elog(ERROR, "missing row in percentile_cont");
 
-				rownum = target_row;
+				rownum = first_row;
+				/* Always advance second_val to be latest input value */
+				second_val = first_val;
 			}
-			else
+			else if (first_row == rownum)
 			{
 				/*
-				 * We are already at the target row, so we must previously
-				 * have read its value into second_val.
+				 * We are already at the desired row, so we must previously
+				 * have read its value into second_val (and perhaps first_val
+				 * as well, but this assignment is harmless in that case).
 				 */
 				first_val = second_val;
 			}
 
 			/* Fetch second_row if needed */
-			if (need_lerp)
+			if (second_row > rownum)
 			{
 				if (!tuplesort_getdatum(osastate->sortstate, true, &second_val, &isnull) || isnull)
 					elog(ERROR, "missing row in percentile_cont");
 				rownum++;
 			}
-			else
-				second_val = first_val;
+			/* We should now certainly be on second_row exactly */
+			Assert(second_row == rownum);
 
 			/* Compute appropriate result */
-			if (need_lerp)
+			if (second_row > first_row)
 				result_datum[idx] = lerpfunc(first_val, second_val,
 											 pct_info[i].proportion);
 			else

src/test/regress/expected/aggregates.out

@@ -1423,11 +1423,11 @@ from tenk1;
  {{NULL,999,499},{749,249,NULL}}
 (1 row)
 
-select percentile_cont(array[0,1,0.25,0.75,0.5,1]) within group (order by x)
+select percentile_cont(array[0,1,0.25,0.75,0.5,1,0.3,0.32,0.35,0.38,0.4]) within group (order by x)
 from generate_series(1,6) x;
-    percentile_cont    
------------------------
- {1,6,2.25,4.75,3.5,6}
+             percentile_cont              
+------------------------------------------
+ {1,6,2.25,4.75,3.5,6,2.5,2.6,2.75,2.9,3}
 (1 row)
 
 select ten, mode() within group (order by string4) from tenk1 group by ten;

src/test/regress/sql/aggregates.sql

@@ -533,7 +533,7 @@ select percentile_cont(array[0,0.25,0.5,0.75,1]) within group (order by thousand
 from tenk1;
 select percentile_disc(array[[null,1,0.5],[0.75,0.25,null]]) within group (order by thousand)
 from tenk1;
-select percentile_cont(array[0,1,0.25,0.75,0.5,1]) within group (order by x)
+select percentile_cont(array[0,1,0.25,0.75,0.5,1,0.3,0.32,0.35,0.38,0.4]) within group (order by x)
 from generate_series(1,6) x;
 
 select ten, mode() within group (order by string4) from tenk1 group by ten;