MERGE_JOIN nil comparison found in operand

[max-hoffman]

Triggering query below, better repro soon. This one seems kind of strange, we explicitly check for and handle ErrNilOperand in Equals, RegExp, GreaterThan, GreaterThanOrEqual, LessThan, and LessThanOrEqual. As far as I can tell, there are no unhandled filters in the queries below. The error type check may be failing.

SELECT `ipam_prefix`.`id`, `ipam_prefix`.`created`, `ipam_prefix`.`last_updated`, `ipam_prefix`.`_custom_field_data`, `ipam_prefix`.`status_id`, `ipam_prefix`.`network`, `ipam_prefix`.`broadcast`, `ipam_prefix`.`prefix_length`, `ipam_prefix`.`site_id`, `ipam_prefix`.`location_id`, `ipam_prefix`.`vrf_id`, `ipam_prefix`.`tenant_id`, `ipam_prefix`.`vlan_id`, `ipam_prefix`.`role_id`, `ipam_prefix`.`is_pool`, `ipam_prefix`.`description`
FROM `ipam_prefix`
LEFT OUTER JOIN `ipam_vrf`
ON (`ipam_prefix`.`vrf_id` = `ipam_vrf`.`id`)
WHERE `ipam_prefix`.`prefix_length` = 15
ORDER BY `ipam_vrf`.`name` ASC, `ipam_prefix`.`network` ASC, `ipam_prefix`.`prefix_length` ASC

error: nil comparison found in operand

[max-hoffman]

This is a merge join bug, mergeJoinIter.peekMatch calls Compare without checking for a nil error:

diff --git a/sql/plan/merge_join.go b/sql/plan/merge_join.go
index 9f5910073..84287fa76 100644
--- a/sql/plan/merge_join.go
+++ b/sql/plan/merge_join.go
@@ -458,7 +458,10 @@ func (i *mergeJoinIter) peekMatch(ctx *sql.Context, iter sql.RowIter) (bool, sql
        // check if lookahead valid
        copySubslice(i.fullRow, peek, off)
        res, err := i.cmp.Compare(ctx, i.fullRow)
-       if err != nil {
+       if expression.ErrNilOperand.Is(err) {
+               // revert change to output row if no match
+               copySubslice(i.fullRow, restore, off)
+       } else if err != nil {
                return false, nil, err
        }

peekMatch is only called immediately after we have a merge join match, so reaching this code is not trivial. We need the indexes to have a matching row, and then immediately have a null field.