vtexplain crash on multiple INNER JOINs

[mdkent]

Overview of the Issue

vtexplain crashes on multiple INNER JOINs

Reproduction Steps

Given the following:

schema.sql

CREATE TABLE `accounts` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

CREATE TABLE `contacts` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `account_id` bigint(20) NOT NULL,
  `contactable_type` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL,
  `contactable_id` bigint(20) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

CREATE TABLE `users` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `account_id` bigint(20) NOT NULL,
  `identity_id` bigint(20) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

and vschema.json

{
  "main": {
    "sharded": true,
    "vindexes": {
      "hash": {
        "type": "hash"
      }
    },
    "tables": {
      "accounts": {
        "columnVindexes": [
          {
            "column": "id",
            "name": "hash"
          }
        ],
        "autoIncrement": {
          "column": "id",
          "sequence": "accounts_seq"
        }
      },
      "contacts": {
        "columnVindexes": [
          {
            "column": "account_id",
            "name": "hash"
          }
        ],
        "autoIncrement": {
          "column": "id",
          "sequence": "contacts_seq"
        }
      },
      "users": {
        "columnVindexes": [
          {
            "column": "account_id",
            "name": "hash"
          }
        ],
        "autoIncrement": {
          "column": "id",
          "sequence": "users_seq"
        }
      }
    }
  },
  "mainunsharded": {
    "tables": {
      "accounts_seq": {
        "type": "sequence"
      },
      "contacts_seq": {
        "type": "sequence"
      },
      "users_seq": {
        "type": "sequence"
      }
    }
  }
}  

Run

vtexplain -vschema-file vschema.json -schema-file schema.sql -output-mode text \
  -sql 'SELECT 1 FROM `accounts` INNER JOIN `contacts` ON `accounts`.`id` = `contacts`.`account_id` INNER JOIN `users` ON `contacts`.`contactable_id` = `users`.`id` LIMIT 1'

and see it crash with

E1020 11:16:47.466618   47768 exit.go:89] panic: runtime error: index out of range [0] with length 0
/usr/local/Cellar/go/1.15.2/libexec/src/runtime/panic.go:969 (0x1038a14)
	gopanic: done = runOpenDeferFrame(gp, d)
/usr/local/Cellar/go/1.15.2/libexec/src/runtime/panic.go:88 (0x1036704)
	goPanicIndex: panic(boundsError{x: int64(x), signed: true, y: y, code: boundsIndex})
/Users/mkent/go/src/vitess.io/vitess/go/vt/vtgate/engine/join.go:183 (0x17e4519)
	io/vitess/go/vt/vtgate/engine.joinFields: fields[i] = lfields[-index-1]
/Users/mkent/go/src/vitess.io/vitess/go/vt/vtgate/engine/join.go:67 (0x17e3d44)
	io/vitess/go/vt/vtgate/engine.(*Join).Execute: result.Fields = joinFields(lresult.Fields, rresult.Fields, jn.Cols)
/Users/mkent/go/src/vitess.io/vitess/go/vt/vtgate/engine/limit.go:68 (0x17e530d)
	io/vitess/go/vt/vtgate/engine.(*Limit).Execute: result, err := l.Input.Execute(vcursor, bindVars, wantfields)
/Users/mkent/go/src/vitess.io/vitess/go/vt/vtgate/plan_execute.go:155 (0x194c89a)
	io/vitess/go/vt/vtgate.(*Executor).executePlan.func1: qr, err := plan.Instructions.Execute(vcursor, bindVars, true)
/Users/mkent/go/src/vitess.io/vitess/go/vt/vtgate/plan_execute.go:100 (0x1934cdc)
	io/vitess/go/vt/vtgate.(*Executor).newExecute: return e.executePlan(ctx, plan, vcursor, bindVars, execStart)(logStats, safeSession)
/Users/mkent/go/src/vitess.io/vitess/go/vt/vtgate/executor.go:176 (0x192269a)
	io/vitess/go/vt/vtgate.(*Executor).execute: stmtType, qr, err := e.newExecute(ctx, safeSession, sql, bindVars, logStats)
/Users/mkent/go/src/vitess.io/vitess/go/vt/vtgate/executor.go:147 (0x19223a4)
	io/vitess/go/vt/vtgate.(*Executor).Execute: stmtType, result, err := e.execute(ctx, safeSession, sql, bindVars, logStats)
/Users/mkent/go/src/vitess.io/vitess/go/vt/vtexplain/vtexplain_vtgate.go:134 (0x1adde89)
	io/vitess/go/vt/vtexplain.vtgateExecute: _, err := vtgateExecutor.Execute(context.Background(), "VtexplainExecute", vtgate.NewSafeSession(vtgateSession), sql, nil)
/Users/mkent/go/src/vitess.io/vitess/go/vt/vtexplain/vtexplain.go:280 (0x1adb8d8)
	io/vitess/go/vt/vtexplain.explain: plans, tabletActions, err := vtgateExecute(sql)
/Users/mkent/go/src/vitess.io/vitess/go/vt/vtexplain/vtexplain.go:263 (0x1adb645)
	io/vitess/go/vt/vtexplain.Run: e, err := explain(sql)
/Users/mkent/go/src/vitess.io/vitess/go/cmd/vtexplain/vtexplain.go:172 (0x1ae7892)
	parseAndRun: plans, err := vtexplain.Run(sql)
/Users/mkent/go/src/vitess.io/vitess/go/cmd/vtexplain/vtexplain.go:132 (0x1ae73b1)
	main: err := parseAndRun()
/usr/local/Cellar/go/1.15.2/libexec/src/runtime/proc.go:204 (0x103b8c8)
	main: fn()
/usr/local/Cellar/go/1.15.2/libexec/src/runtime/asm_amd64.s:1374 (0x1071980)
	goexit: BYTE	$0x90	// NOP

The second JOIN seems to do it, as this passes:

$ vtexplain -vschema-file vschema.json -schema-file schema.sql -output-mode text -sql 'SELECT 1 AS one FROM `accounts` INNER JOIN `contacts` ON `accounts`.`id` = `contacts`.`account_id` LIMIT 1'
----------------------------------------------------------------------
SELECT 1 AS one FROM `accounts` INNER JOIN `contacts` ON `accounts`.`id` = `contacts`.`account_id` LIMIT 1

1 main/-80: select 1 as one from accounts join contacts on accounts.id = contacts.account_id limit 1
1 main/80-: select 1 as one from accounts join contacts on accounts.id = contacts.account_id limit 1

----------------------------------------------------------------------

Binary version

Installed via go get -v vitess.io/vitess/go/cmd/vtexplain but the version looks broken:

Version:  (Git branch '') built on  by @ using go1.15.2 darwin/amd64

Operating system and Environment details

OSX 10.15.6