sql: fix decimal evaluation edge cases

Previously, the logic for decimal and float division, floor division
and mod operators was incorrect for a few edge cases involving `NaN`
or `Infinity` values. For example, `'NaN'::DECIMAL / 0` would throw
a division-by-zero error when it should evaluate to `NaN` and
`0/'inf'::DECIMAL` returned `0E-2019` instead of just `0`.

This patch updates the special-case logic to mirror that of postgres,
so division-by-zero errors always check the `NaN` case and the division
by infinity case returns a `0` without extra digits.

Fixes #40929
Fixes #103633

Release note (bug fix): Fixed edge cases in decimal and float evaluation
for division operators. `'NaN'::DECIMAL / 0` will now return `NaN` instead
of a division-by-zero error, and `0 / 'inf'::DECIMAL` will return `0`
instead of `0E-2019`.

pkg/cmd/roachtest/tests/sqlsmith.go

@@ -231,18 +231,8 @@ WITH into_db = 'defaultdb', unsafe_restore_incompatible_version;
 			if err != nil {
 				es := err.Error()
 				if strings.Contains(es, "internal error") {
-					// TODO(yuzefovich): we temporarily ignore internal errors
-					// that are because of #40929.
-					var expectedError bool
-					for _, exp := range []string{
-						"could not parse \"0E-2019\" as type decimal",
-					} {
-						expectedError = expectedError || strings.Contains(es, exp)
-					}
-					if !expectedError {
-						logStmt(stmt)
-						t.Fatalf("error: %s\nstmt:\n%s;", err, stmt)
-					}
+					logStmt(stmt)
+					t.Fatalf("error: %s\nstmt:\n%s;", err, stmt)
 				} else if strings.Contains(es, "Empty statement returned by generate") ||
 					stmt == "" {
 					// Either were unable to generate a statement or