SQL COUNT() function with DISTINCT clause eliminates the repetitive appearance of the same data. NB: These techniques are universal, but for syntax we chose Postgres. 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. Cette requête est assez lente. 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; Distributed Distinct Count with HyperLogLog on Postgres¶ (Copy of original publication) Running SELECT COUNT(DISTINCT) on your database is all too common. 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. Thanks to the inimitable pgAdminIII for the Explain graphics.. Which, in turn, was also commented on Reddit. 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 . So Useful, Yet So Slow. Double colon (: ð notation en SQL; Comment supprimer des entrées en double? 06.. 34629. So, couple of days ago, some guy, from Periscope company wrote a blogpost about getting number of distinct elements, per group, faster using subqueries.. The docs explain DISTINCT ON: SELECT DISTINCT ON ( expression [, â¦] ) keeps only the first row of each set of rows where the given expressions evaluate to equal. The DISTINCT can come only once in a given select statement. Mise à jour du nom de la contrainte dans PostgreSQL As many blog posts mention (âone weird trick to make postgres 50x faster!â) 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. Syntax : COUNT(DISTINCT expr,[expr...]) Example : To get unique number of rows from the 'orders' table with following conditions - 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: il faut environ 7,5 secondes, comparé à . 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. SELECT COUNT(DISTINCT x) FROM table; Ma table compte environ 1,5 million de lignes. In applications itâs typical to have some analytics dashboard highlighting the number of unique items such as unique users, unique products, unique visits. 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. 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. test=> select distinct col_int from t1; Time: 1264.667 ms test=> select distinct col_int from t1; Time: 1261.805 ms. My table only has two columns- user_id (there can â¦ COUNT() function with distinct clause . PostgreSQL does all the heavy lifting for us. And the reason I haven't heard about it is: Nonstandard Clauses DISTINCT ON ( â¦ ) is an extension of the SQL standard. PostgreSQL does not have it yet but has it on the roadmap as of 2020. , in turn postgres select distinct count was also commented on Reddit select DISTINCT operator is on a PostGIS geometry COUNT )... First time users per day Ma table compte environ 1,5 million de lignes clause eliminates the repetitive appearance of same... 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