Trigger to invalidate cache after update - mysql

Question

I'm trying to write a trigger to invalidate the word index for my story database. However I can't seem to figure out how to stop the trigger from firing again during the indexing operation. I know I need to place an if statement to stop the update, but I can't seem to figure out what it should look like.
CREATE TRIGGER trMarkStoryForReindex BEFORE UPDATE ON Chapters
FOR EACH ROW BEGIN
-- any update to any chapter invalidates the index for the whole story
-- we could just periodically flush the story index, but this way is
-- better.
SET New.isIndexed = FALSE;
-- delete the index for that story
-- It will get rebuilt in at most 15 minutes
DELETE FROM freq WHERE storyid = NEW.StoryId;
END;
I basically want the trigger to fire only when isIndexed is has not been set in the update statement causing the trigger.
My data model looks like so:
Chapters
id
isIndexed
StoryId
Freq
word
storyid

Answer 1

Here's my proposal for solution. I've tested this on SQL fiddle, and it seems to work:
-- Database setup
create table chapters (
id int unsigned not null auto_increment primary key,
isIndexed boolean default false,
storyId int not null,
index idx_storyId(storyId)
);
create table freq (
word varchar(50),
storyId int not null,
index idx_storyId(storyId)
);
delimiter //
create trigger bi_chapters before update on chapters
for each row
begin
if new.isIndexed = false then
delete from freq where storyId = new.storyId;
end if;
end //
delimiter ;
insert into freq(word, storyId)
values ('one', 1), ('two', 1), ('three', 2);
insert into chapters(isIndexed, storyId)
values (true, 1), (true, 2);
When you select the values from freq (before updating chapters) you get this:
select * from chapters;
| id | isIndexed | storyId |
|----|-----------|---------|
| 1 | false | 1 |
| 2 | true | 2 |
select * from freq;
| word | storyId |
|-------|---------|
| one | 1 |
| two | 1 |
| three | 2 |
Now, do an update to chapters and select from freq again:
update chapters
set isIndexed = false
where storyId = 1;
select * from freq;
| word | storyId |
|-------|---------|
| three | 2 |
The only modification I did is that if block that checks if the new row is updated to false. If I've understood your question correctly, this would do what you need.
SQL fiddle example