Matthew Wilson wrote:
> I have a table MESSAGE with a text column and a timestamp column. I
> want to make sure that I have no rows with duplicates for both values.
> I have two ideas right now for how to do this.
>
> IDEA 1:
>
> CREATE UNIQUE INDEX nodups on MESSAGE (my_timestamp_col, my_text_col);
>
> IDEA 2:
>
> CREATE UNIQUE INDEX nodups on MESSAGE (my_timestamp_col, md5(my_text_col));
>
> I am speculating that an index on the md5 is cheaper than on a text
> column. I'm willing to risk the chance of a hash collision.
>
> I don't want to use this index to allow searching inside the text
> column. I just want to protect against duplication.
>
> Are either of these approaches any good? What are other ways to
> guarantee uniqueness for the pair of my timestamp column and my text
> column?
Idea 1 will only work if your text columns never grow too long - I can't
recall what the limit is off the top of my head but PG will issue warnings
when you reach it.
Idea 2 works. You can even use it for exact match searching if you like if
you do WHERE my_timestamp_col='...' AND md5(my_text_col)=md5('...')
Depending on your app, you might want to round your timestamps to avoid
getting two identical messages in the system only a millisecond apart.
--
Stuart Bishop <stuart@stuartbishop.net>
http://www.stuartbishop.net/