postgres select distinct count

The DISTINCT can come only once in a given select statement. Syntax : COUNT(DISTINCT expr,[expr...]) Example : To get unique number of rows from the 'orders' table with following conditions - PostgreSQL does not have it yet but has it on the roadmap as of 2020. So Useful, Yet So Slow. 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. test=> select distinct col_int from t1; Time: 1264.667 ms test=> select distinct col_int from t1; Time: 1261.805 ms. 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. And the reason I haven't heard about it is: Nonstandard Clauses DISTINCT ON ( â¦ ) is an extension of the SQL standard. Which, in turn, was also commented on Reddit. 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. SQL COUNT() function with DISTINCT clause eliminates the repetitive appearance of the same data. 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 . Distributed Distinct Count with HyperLogLog on Postgres¶ (Copy of original publication) Running SELECT COUNT(DISTINCT) on your database is all too common. 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. Double colon (: ð notation en SQL; Comment supprimer des entrées en double? Mise à jour du nom de la contrainte dans PostgreSQL Cette requête est assez lente. 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; 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. 06.. 34629. COUNT() function with distinct clause . NB: These techniques are universal, but for syntax we chose Postgres. SELECT COUNT(DISTINCT x) FROM table; Ma table compte environ 1,5 million de lignes. il faut environ 7,5 secondes, comparé à . My table only has two columns- user_id (there can â¦ 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 itâs typical to have some analytics dashboard highlighting the number of unique items such as unique users, unique products, unique visits. 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. Count distinct is the bane of SQL analysts, so it was an obvious choice for our first blog post. 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