fix NaN cost estimate on empty columnar tables (#4593)

Fixing a division by zero in the cost calculations for scanning a columnar table.

Due to how the columns in a columnar table are counted an empty table would result in a division by zero. Instead this patch keeps the column selection ratio on zero when this happens, resulting in an accurate cost of zero pages to scan a columnar table.

fixes #4589

src/backend/columnar/cstore_customscan.c

@@ -242,7 +242,18 @@ ColumnarScanCost(RangeTblEntry *rte)
 	{
 		Bitmapset *attr_needed = rte->selectedCols;
 		double numberOfColumnsRead = bms_num_members(attr_needed);
-		double selectionRatio = numberOfColumnsRead / (double) maxColumnCount;
+		double selectionRatio = 0;
+
+		/*
+		 * When no stripes are in the table we don't have a count in maxColumnCount. To
+		 * prevent a division by zero turning into a NaN we keep the ratio on zero.
+		 * This will result in a cost of 0 for scanning the table which is a reasonable
+		 * cost on an empty table.
+		 */
+		if (maxColumnCount != 0)
+		{
+			selectionRatio = numberOfColumnsRead / (double) maxColumnCount;
+		}
 		Cost scanCost = (double) totalStripeSize / BLCKSZ * selectionRatio;
 		return scanCost;
 	}

src/test/regress/expected/am_empty.out

@@ -94,6 +94,19 @@ truncate t_compressed;
 -- alter type
 alter table t_uncompressed alter column a type text;
 alter table t_compressed alter column a type text;
+-- verify cost of scanning an empty table is zero, not NaN
+explain table t_uncompressed;
+                                   QUERY PLAN
+---------------------------------------------------------------------
+ Custom Scan (ColumnarScan) on t_uncompressed  (cost=0.00..0.00 rows=1 width=32)
+(1 row)
+
+explain table t_compressed;
+                                  QUERY PLAN
+---------------------------------------------------------------------
+ Custom Scan (ColumnarScan) on t_compressed  (cost=0.00..0.00 rows=1 width=32)
+(1 row)
+
 -- drop
 drop table t_compressed;
 drop table t_uncompressed;

src/test/regress/sql/am_empty.sql

@@ -43,6 +43,10 @@ truncate t_compressed;
 alter table t_uncompressed alter column a type text;
 alter table t_compressed alter column a type text;
 
+-- verify cost of scanning an empty table is zero, not NaN
+explain table t_uncompressed;
+explain table t_compressed;
+
 -- drop
 drop table t_compressed;
 drop table t_uncompressed;