Don't lose precision for float fields of Nodes.

Historically we've been more worried about making the output of
float fields look pretty than whether they'd be read back exactly.
That won't work if we're to compare the read-back nodes for
equality, so switch to using the Ryu code for float output.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/4159834.1657405226@sss.pgh.pa.us

2 files changed, 25 insertions, 10 deletions

diff --git a/src/backend/nodes/gen_node_support.pl b/src/backend/nodes/gen_node_support.pl
index b707a09f565..81b8c184a90 100644
--- a/src/backend/nodes/gen_node_support.pl
+++ b/src/backend/nodes/gen_node_support.pl
@@ -983,29 +983,29 @@ _read${n}(void)
 		}
 		elsif ($t eq 'double')
 		{
-			print $off "\tWRITE_FLOAT_FIELD($f, \"%.6f\");\n";
+			print $off "\tWRITE_FLOAT_FIELD($f);\n";
 			print $rff "\tREAD_FLOAT_FIELD($f);\n" unless $no_read;
 		}
 		elsif ($t eq 'Cardinality')
 		{
-			print $off "\tWRITE_FLOAT_FIELD($f, \"%.0f\");\n";
+			print $off "\tWRITE_FLOAT_FIELD($f);\n";
 			print $rff "\tREAD_FLOAT_FIELD($f);\n" unless $no_read;
 		}
 		elsif ($t eq 'Cost')
 		{
-			print $off "\tWRITE_FLOAT_FIELD($f, \"%.2f\");\n";
+			print $off "\tWRITE_FLOAT_FIELD($f);\n";
 			print $rff "\tREAD_FLOAT_FIELD($f);\n" unless $no_read;
 		}
 		elsif ($t eq 'QualCost')
 		{
-			print $off "\tWRITE_FLOAT_FIELD($f.startup, \"%.2f\");\n";
-			print $off "\tWRITE_FLOAT_FIELD($f.per_tuple, \"%.2f\");\n";
+			print $off "\tWRITE_FLOAT_FIELD($f.startup);\n";
+			print $off "\tWRITE_FLOAT_FIELD($f.per_tuple);\n";
 			print $rff "\tREAD_FLOAT_FIELD($f.startup);\n"   unless $no_read;
 			print $rff "\tREAD_FLOAT_FIELD($f.per_tuple);\n" unless $no_read;
 		}
 		elsif ($t eq 'Selectivity')
 		{
-			print $off "\tWRITE_FLOAT_FIELD($f, \"%.4f\");\n";
+			print $off "\tWRITE_FLOAT_FIELD($f);\n";
 			print $rff "\tREAD_FLOAT_FIELD($f);\n" unless $no_read;
 		}
 		elsif ($t eq 'char*')
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 63dda75ae52..64c65f060b4 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -17,6 +17,7 @@
 #include <ctype.h>
 
 #include "access/attnum.h"
+#include "common/shortest_dec.h"
 #include "lib/stringinfo.h"
 #include "miscadmin.h"
 #include "nodes/bitmapset.h"
@@ -25,6 +26,7 @@
 #include "utils/datum.h"
 
 static void outChar(StringInfo str, char c);
+static void outDouble(StringInfo str, double d);
 
 
 /*
@@ -69,9 +71,10 @@ static void outChar(StringInfo str, char c);
 	appendStringInfo(str, " :" CppAsString(fldname) " %d", \
 					 (int) node->fldname)
 
-/* Write a float field --- caller must give format to define precision */
-#define WRITE_FLOAT_FIELD(fldname,format) \
-	appendStringInfo(str, " :" CppAsString(fldname) " " format, node->fldname)
+/* Write a float field (actually, they're double) */
+#define WRITE_FLOAT_FIELD(fldname) \
+	(appendStringInfo(str, " :" CppAsString(fldname) " "), \
+	 outDouble(str, node->fldname))
 
 /* Write a boolean field */
 #define WRITE_BOOL_FIELD(fldname) \
@@ -199,6 +202,18 @@ outChar(StringInfo str, char c)
 }
 
 /*
+ * Convert a double value, attempting to ensure the value is preserved exactly.
+ */
+static void
+outDouble(StringInfo str, double d)
+{
+	char		buf[DOUBLE_SHORTEST_DECIMAL_LEN];
+
+	double_to_shortest_decimal_buf(d, buf);
+	appendStringInfoString(str, buf);
+}
+
+/*
  * common implementation for scalar-array-writing functions
  *
  * The data format is either "<>" for a NULL pointer or "(item item item)".
@@ -525,7 +540,7 @@ _outRangeTblEntry(StringInfo str, const RangeTblEntry *node)
 			break;
 		case RTE_NAMEDTUPLESTORE:
 			WRITE_STRING_FIELD(enrname);
-			WRITE_FLOAT_FIELD(enrtuples, "%.0f");
+			WRITE_FLOAT_FIELD(enrtuples);
 			WRITE_OID_FIELD(relid);
 			WRITE_NODE_FIELD(coltypes);
 			WRITE_NODE_FIELD(coltypmods);