Hacker News Reader

Comment by dotancohen | Hacker News Reader

I've been told similar nasty things for adding LIMIT 1 to queries that I expect to return at most a single result, such as querying for an ID. But on large tables (at least in sqlite, mysql, and maybe postgress too) the database will continue to search the entire table after the given record was found.

View on HN

Only if your table is missing an unique index on that column, which it should have to enforce your assumption, so yeah LIMIT 1 is a code (or schema in the case) smell.

IDs are typically unique primary key. But in my experience, adding LIMIT 1 would on average halve the time taken to retrieve the record.

I'll test again, really the last time I tested that was two decades ago.

You are certainly doing something wrong if that's true.

I'm curious, can you demo this?

I'm curious as well to see if this still holds up. I'll try this week.

That seems like your RDBMS wasn't handling something right there or there wasn't a unique index on the column.

Do you recall what the database server was?

Yes, I was using Mysql exclusively at the time. I don't recall which version.

I also tested this once years later when doing a Python app with sqlite. Similar result, but admittedly that was not a very big table to begin with.

I am meticulous with my database schemas, and periodically review my indexes and covering indexes. I'm no DBA, but I believe that the database is the only real value a codebase has, other than maybe a novel method here and there. So I put care into designing it properly and testing my assumptions.

That would be a reportable bug. Of a pretty high priority.

You should use the DB explain or equivalent command to spit out the query plan, limit 1 shouldn't change anything in your case, if it's not the case you should file an issue, it's pretty much 101 of query optimization.

I've noticed that LIMIT 1 makes a huge difference when working with LATERAL JOINs in Postgres, even when the WHERE condition has a unique constraint.

If you include an ORDER BY, the DB _may_ continue searching. MySQL (and, I assume, MS SQL Server, since it also can cluster the PK) can stop early in some circumstances.

But if you just have a LIMIT, then no - any RDBMS should stop as soon as it’s reached your requested limit.

Right, that's why I add it.

In mysql, the db will continue reading even if the limit condition has been met, and then anything beyond the limit will be discarded before returning the result.

Even without an ORDER BY clause?

Nope, that does work as expected, unless a filesort is required, good point.

Reply on news.ycombinator.com