SET @preparedStatement = (SELECT IF(
    NOT EXISTS(
        SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS
        WHERE table_name = 'Reactions'
        AND table_schema = DATABASE()
        AND column_name = 'ChannelId'
    ),
    'ALTER TABLE Reactions ADD COLUMN ChannelId varchar(26) NOT NULL DEFAULT "";',
    'SELECT 1;'
));

PREPARE addColumnIfNotExists FROM @preparedStatement;
EXECUTE addColumnIfNotExists;
DEALLOCATE PREPARE addColumnIfNotExists;


UPDATE Reactions SET ChannelId = COALESCE((select ChannelId from Posts where Posts.Id = Reactions.PostId), '') WHERE ChannelId="";


SET @preparedStatement = (SELECT IF(
    (
        SELECT COUNT(*) FROM INFORMATION_SCHEMA.STATISTICS
        WHERE table_name = 'Reactions'
        AND table_schema = DATABASE()
        AND index_name = 'idx_reactions_channel_id'
    ) > 0,
    'SELECT 1',
    'CREATE INDEX idx_reactions_channel_id ON Reactions(ChannelId);'
));

PREPARE createIndexIfNotExists FROM @preparedStatement;
EXECUTE createIndexIfNotExists;
DEALLOCATE PREPARE createIndexIfNotExists;