For others seeking speeding up SELECT DISTINCT without WHERE: Some database engines implement a special algorithm ("index skip scan", "loose indexscan", "jump scan") just to select distinct values from the leading columns of a b-tree index. In applications it’s typical to have some analytics dashboard highlighting the number of unique items such as unique users, unique products, unique visits. il faut environ 7,5 secondes, comparé à . Howdy, Michelle, If you write something like this, SELECT hour , COUNT(id_user) as count FROM ( SELECT EXTRACT(hour from my_date) as hour, id_user FROM michelle GROUP BY EXTRACT(hour from my_date),id_user )subquery GROUP BY hour for each hour it will count the number of distinct user_id's there are . Better explained in this blog post but basically, cutting to the chase, here's how you count on an indexed field: songsearch=# select count(*) from (select distinct text_hash from main_songtexthash) t; count ----- 1825983 (1 row) And the explanation and cost analysis is: The DISTINCT can come only once in a given select statement. So Useful, Yet So Slow. NB: These techniques are universal, but for syntax we chose Postgres. Distributed Distinct Count with HyperLogLog on Postgres¶ (Copy of original publication) Running SELECT COUNT(DISTINCT) on your database is all too common. SELECT COUNT(DISTINCT x) FROM table; Ma table compte environ 1,5 million de lignes. Syntax : COUNT(DISTINCT expr,[expr...]) Example : To get unique number of rows from the 'orders' table with following conditions - And the reason I haven't heard about it is: Nonstandard Clauses DISTINCT ON ( … ) is an extension of the SQL standard. 06.. 34629. COUNT() function with distinct clause . Cette requête est assez lente. SQL COUNT() function with DISTINCT clause eliminates the repetitive appearance of the same data. test=> select distinct col_int from t1; Time: 1264.667 ms test=> select distinct col_int from t1; Time: 1261.805 ms. This was then submitted to Hacker News and r/Programming on Reddit.. Then, the original authors submitted second blogpost comparing speed between four different DB engines. Which, in turn, was also commented on Reddit. On my system, the following query gives me a count of 5, which means that the inserted points are considered equal if they differ by less than 1e-5 and I am not sure if that is a feature of PostGIS, a problem of my installation or a bug. So, couple of days ago, some guy, from Periscope company wrote a blogpost about getting number of distinct elements, per group, faster using subqueries.. I think you might struggle to do this with window functions simply because postgres doesn't seem to support the distinct clause within a count, which is what you need to prevent users being double-counted within the 31 day frame. Thanks to the inimitable pgAdminIII for the Explain graphics.. I am new topsql and am interested in counting the number of unique first time users per day. I wonder what the precision of the SELECT DISTINCT operator is on a PostGIS geometry. PostgreSQL does all the heavy lifting for us. If I use "group by": test=> select distinct col_int from t1 group by col_int; Time: 1180.617 ms test=> select distinct col_int from t1 group by col_int; Time: 1179.849 ms test=> select distinct col_int from t1 group by col_int; As many blog posts mention (“one weird trick to make postgres 50x faster!”) you can guide the planner by rewriting count distinct as the count of a subquery:-- SELECT COUNT(DISTINCT n) FROM items; -- must be rewritten as EXPLAIN SELECT COUNT (*) FROM (SELECT DISTINCT n FROM items) t; Aggregate (cost = 34629. Count distinct is the bane of SQL analysts, so it was an obvious choice for our first blog post. Double colon (: 🙂 notation en SQL; Comment supprimer des entrées en double? PostgreSQL does not have it yet but has it on the roadmap as of 2020. 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