sql/pgwire: fix precision loss in binary decimal decoding

Decimal decoding implementation was ignoring trailing
zeros after the decimal point that were implicitly added
by the dscale value, resulting precision losses. For
example, 1.23000 could be decoded as 1.23. This commit
fixes it by adding missing zeros.

Release note: None
Epic: CRDB-57092
Fixes: #158093

Author: dils2k

pkg/sql/pgwire/pgwirebase/encoding.go

@@ -655,9 +655,19 @@ func DecodeDatum(
 				// is larger than our calculated dscale, truncate our buffer to match the
 				// desired dscale.
 				dscale := (alloc.pgNum.Ndigits - (alloc.pgNum.Weight + 1)) * PGDecDigits
+
 				if overScale := dscale - alloc.pgNum.Dscale; overScale > 0 {
 					dscale -= overScale
 					decDigits = decDigits[:len(decDigits)-int(overScale)]
+				} else if overScale < 0 {
+					z := -overScale
+					if dscale < 0 {
+						z = alloc.pgNum.Dscale
+					}
+					for i := int16(0); i < z; i++ {
+						decDigits = append(decDigits, '0')
+						dscale++
+					}
 				}
 
 				decString := encoding.UnsafeConvertBytesToString(decDigits)

pkg/sql/pgwire/testdata/pgtest/decimal

@@ -81,3 +81,40 @@ ReadyForQuery
 {"Type":"DataRow","Values":[{"binary":"00010001000000000001"},{"binary":"00010001000000000001"},{"binary":"0001000100000000000a"}]}
 {"Type":"CommandComplete","CommandTag":"SELECT 1"}
 {"Type":"ReadyForQuery","TxStatus":"I"}
+
+# ResultFormatCodes [1] = FormatBinary
+# [0, 2, 0, 0, 0, 0, 0, 5, 0, 1, 8fc] = binary 1.23000
+# [0, 2, 0, 0, 0, 0, 0, a, 0, 1, 8fc] = binary 1.2300000000
+# [0, 1, 0, 0, 0, 0, 0, a, 0, 1] = binary 1.0000000000
+send
+Parse {"Name": "s3", "Query": "SELECT $1::decimal, $2::decimal, $3::decimal"}
+Bind {"DestinationPortal": "p3", "PreparedStatement": "s3", "ParameterFormatCodes": [1], "Parameters": [{"binary":"0002000000000005000108fc"}, {"binary":"000200000000000a000108fc"}, {"binary": "000100000000000a0001"}]}
+Execute {"Portal": "p3"}
+Sync
+----
+
+until
+ReadyForQuery
+----
+{"Type":"ParseComplete"}
+{"Type":"BindComplete"}
+{"Type":"DataRow","Values":[{"text":"1.23000"},{"text":"1.2300000000"},{"text":"1.0000000000"}]}
+{"Type":"CommandComplete","CommandTag":"SELECT 1"}
+{"Type":"ReadyForQuery","TxStatus":"I"}
+
+# [0, 1, 0, 1, 0, 0, 0, 3, 0, 1] = binary 1.000E+4
+send
+Parse {"Name": "s4", "Query": "SELECT $1::decimal"}
+Bind {"DestinationPortal": "p4", "PreparedStatement": "s4", "ParameterFormatCodes": [1], "Parameters": [{"binary":"00010001000000030001"}]}
+Execute {"Portal": "p4"}
+Sync
+----
+
+until
+ReadyForQuery
+----
+{"Type":"ParseComplete"}
+{"Type":"BindComplete"}
+{"Type":"DataRow","Values":[{"text":"1.000E+4"}]}
+{"Type":"CommandComplete","CommandTag":"SELECT 1"}
+{"Type":"ReadyForQuery","TxStatus":"I"}