Does select distinct affect performance?

Yes, as using DISTINCT will (sometimes according to a comment) cause results to be ordered. Sorting hundreds of records takes time. Try GROUP BY all your columns, it can sometimes lead the query optimiser to choose a more efficient algorithm (at least with Oracle I noticed significant performance gain).

What slows down a query?

Slow queries can mean your database does more work than it needs to, which means it’s using more resources than it needs to. When limited resources like CPU or I/O run out, everything can start to slow down. Inefficient use of resources is also a problem when you’re not using the resources you have.

Why you shouldn’t use select distinct?

As a general rule, SELECT DISTINCT incurs a fair amount of overhead for the query. Hence, you should avoid it or use it sparingly. The idea of generating duplicate rows using JOIN just to remove them with SELECT DISTINCT is rather reminiscent of Sisyphus pushing a rock up a hill, only to have it roll back down again.

Is select distinct faster?

However, GROUP BY can produce misleading results, which I think is reason enough for avoiding it. Please see here if you want to measure the time difference between DISTINCT and GROUP BY running your query.

Why is distinct so slow SQL?

It’s slow because the database is iterating over all the logs and all the dashboards, then joining them, then sorting them, all before getting down to real work of grouping and aggregating.

Is it good to use distinct in SQL?

SQL DISTINCT clause is used to remove the duplicates columns from the result set. The distinct keyword is used with select keyword in conjunction. It is helpful when we avoid duplicate values present in the specific columns/tables. The unique values are fetched when we use the distinct keyword.

How slow is a slow query?

The slow queries definition might differ in different cases since there are certain occasions that even a 10 second query is acceptable and still not slow. However, if your application is an OLTP, it’s very common that a 10 second or even a 5 second query is an issue or causes performance degradation to your database.

How do you optimize a slow query?

Supercharge Your SQL Queries for Production Databases

1. Define business requirements first.
2. SELECT fields instead of using SELECT *
3. Avoid SELECT DISTINCT.
4. Create joins with INNER JOIN (not WHERE)
5. Use WHERE instead of HAVING to define filters.
6. Use wildcards at the end of a phrase only.
7. Use LIMIT to sample query results.

When should you use distinct in SQL?

The SELECT DISTINCT statement is used to return only distinct (different) values. Inside a table, a column often contains many duplicate values; and sometimes you only want to list the different (distinct) values.

Is select distinct slower than GROUP BY?

DISTINCT is used to filter unique records out of all records in the table. It removes the duplicate rows. SELECT DISTINCT will always be the same, or faster than a GROUP BY.

Why is distinct slow?

Why DISTINCT queries are slow on PostgreSQL Why are DISTINCT queries slow on PostgreSQL when they seem to ask an “easy” question? It turns out that PostgreSQL currently lacks the ability to efficiently pull a list of unique values from an ordered index.

Is group by faster than distinct Postgres?

From experiments, I founded that the GROUP BY is 10+ times faster than DISTINCT.

Is the distinctportion of the query the reason it runs slowly?

Pretending that I’m 100\% certain the DISTINCTportion of the query is the reason it runs slowly, I’ve omitted the rest of the query to avoid confusion, since it is the distinct portion’s slowness that I’m primarily concerned with (distinct is always a source of slowness). The table in question has 2.5 million rows of data.

How much faster is using select distinct for group by?

Thanks for the great tip Jack! Swapping SELECT DISTINCT for GROUP BY reduced the runtime of my particular query from 649ms down to 87ms, almost 7.5x faster. My INNER JOIN is between a table containing ~30]

When to use distinct in a SQL query?

@orokusaki: it really depends. Frequently, the use of distinct in a query reflects a sub-optimal join somewhere. Not always, but frequently enough. In such cases, the idea is to rewrite the query so that the sub-statement is in a sub-query that returns unique rows (or is checked using the in() clause).

Is it possible to use an index instead of a distinct?

It is not a distinct on a side, a join on an other side, and a group by on the third side. It is not because your query is faster whithout the distinct that the distinct is the problem… You can’t answer a question like that. Or yes you can : use Index. – Cyril Gandon Jul 6 ’11 at 15:40 1 @orokusaki: it really depends.