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When a Federated table is created, access to the remote server is checked.
However, the remote server can become unavailable later. If it happens, the mere existence of the Federated table referring to it can cause troubles for other queries.
In the test case below server restart on a different port imitates disappearance of the remote server as it was configured for the Federated table.
--source include/have_innodb.inc
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install soname 'ha_federatedx';
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create table t1 (a int);
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create database fed;
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--eval create table fed.t2 (a int) engine=federated connection='mysql://root@127.0.0.1:$MASTER_MYPORT/test/t1'
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select * from fed.t2;
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select * from information_schema.referential_constraints;
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--let $port= `SELECT @@port+1`
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--let $restart_parameters= --port=$port
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--source include/restart_mysqld.inc
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select * from information_schema.referential_constraints;
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--let $restart_parameters=
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--source include/restart_mysqld.inc
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On 10.3-10.6 it causes error 2013 upon the query from I_S:
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10.3 1e5ebf37
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query 'select * from information_schema.referential_constraints' failed: 2003: Received error: 2003 : Can't connect to MySQL server on '127.0.0.1' (111 "Connection refused")
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On 10.2 it doesn't fail, but causes a strange warning (same as described in MDEV-25823 for 10.2):
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10.2 d06205ba
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select * from information_schema.referential_constraints;
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CONSTRAINT_CATALOG CONSTRAINT_SCHEMA CONSTRAINT_NAME UNIQUE_CONSTRAINT_CATALOG UNIQUE_CONSTRAINT_SCHEMA UNIQUE_CONSTRAINT_NAME MATCH_OPTION UPDATE_RULE DELETE_RULE TABLE_NAME REFERENCED_TABLE_NAME
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Warnings:
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Warning 1939 Engine PERFORMANCE_SCHEMA failed to discover table `performance_schema`.`accounts` with 'CREATE TABLE accounts(USER CHAR(128) collate utf8_bin default null,HOST CHAR(60) collate utf8_bin default null,CURRENT_CONNECTIONS bigint not null,TOTAL_CONNECTIONS bigint not null)'
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