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

diff --git a/src/backend/nodes/gen_node_support.pl b/src/backend/nodes/gen_node_support.pl
index b707a09f565b1a15b6fcf261dcf1936da2cae4d0..81b8c184a90bb52a3474d547e4a75d73fbf61042 100644 (file)
--- 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 63dda75ae52e72a35c228c2bef6207bb2c01a6b8..64c65f060b4ad786337a6bbaa4e1f9d7ce5828db 100644 (file)
--- 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) \
@@ -198,6 +201,18 @@ outChar(StringInfo str, char c)
    outToken(str, in);
 }
 
+/*
+ * 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
  *
@@ -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);