SHOW FULL FIELDS does not work when keyspace/database name is included

[artemvovk]

Overview of the Issue

When executing SHOW FULL FIELDS command that includes database/keyspace name, Vitess seems to look treat the keyspace.table as the table name, which leads to the command failing. The same query runs successfully on a raw MySQL instance.

Reproduction Steps

Steps to reproduce this issue, example:

1. Deploy the following vschema:

   { "a": {
     "sharded": false,
     "vindexes": {
       "hash": {
         "type": "hash"
       },
     "tables": {
       "table1": {
         "column_vindexes": [
           {
             "column": "user_id",
             "name": "hash"
           }
         ]
       }
     } }, 
     "b": {
     "sharded": false,
     "vindexes": {
       "hash": {
         "type": "hash"
       },
     "tables": {
       "table2": {
         "column_vindexes": [
           {
             "column": "user_id",
             "name": "hash"
           }
         ]
       }
     } }, 
   }

2. Deploy the following schema:

   create database `a`;
   create table `a`.`table1`(user_id bigint, name varchar(128), primary key(user_id));
   create database `b`;
   create table `b`.`table2`(user_id bigint, name varchar(128), primary key(user_id));

3. Run

SHOW FULL FIELDS FROM `a`.`table1`;

1. View error

Mysql2::Error: vtgate: http://9be878f72d30:8080/: target: a.-.master, used tablet: test-101 (vttablet-db-test): vttablet: rpc error: code = NotFound desc = Table 'a.table1' doesn't exist (errno 1146) (sqlstate 42S02) (CallerID: userData1): Sql: "SHOW FULL FIELDS FROM `a`.`table1`", BindVars: {}: SHOW FULL FIELDS FROM `a`.`table1`

Binary version

Example:

Version: b6c7ffcbd (Git branch 'master') built on Tue Jul 14 02:09:57 UTC 2020 by root@f3fe282ca2eb using go1.13.9 linux/amd64

Operating system and Environment details

OS, Architecture, and any other information you can provide
about the environment.

• Operating system (output of cat /etc/os-release): MacOS - Docker
• Kernel version (output of uname -sr): Linux 4.19.76-linuxkit
• Architecture (output of uname -m): x86_64

Log Fragments

Include appropriate log fragments. If the log is longer than a few dozen lines, please
include the URL to the gist of the log instead of posting it in the issue.

VtTablet logs:

Execute	172.24.0.16:54050:/queryservice.Query/Execute(gRPC)	gRPC	'userData1'	'artemavovk'	2020-07-21 20:38:37.180407	2020-07-21 20:38:37.181651	0.001244	OtherRead	"SHOW FULL FIELDS FROM `a`.`table1`"	map[]	1	"SHOW FULL FIELDS FROM `a`.`table1`"	mysql	0.000839	0.000015	0	0	"Table 'a.table1' doesn't exist (errno 1146) (sqlstate 42S02) (CallerID: userData1): Sql: \"SHOW FULL FIELDS FROM `a`.`table1`\", BindVars: {}"	

VtGate logs:

Execute	172.24.0.1:34518	artemavovk	'userData1'	'artemavovk'	2020-07-21 20:34:49.301841	2020-07-21 20:34:49.347320	0.045406	0.000000	0.045302	0.000000	SHOW	"SHOW FULL FIELDS FROM `a`.`table1`"	map[]	1	0	"target: c.-.master, used tablet: test-201 (vttablet-c): vttablet: rpc error: code = NotFound desc = Table 'a.table1' doesn't exist (errno 1146) (sqlstate 42S02) (CallerID: userData1): Sql: \"SHOW FULL FIELDS FROM `a`.`table1`\", BindVars: {}"	"c"	""	"MASTER"	

[harshit-gangal]

Supported via #6825