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Why is my SELECT DISTINCT query so slow?
Pretending that I’m 100% certain the DISTINCT portion 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 to speed up SELECT DISTINCT query in Postgres?
If you put an index on the column (s) selected by the query, the database may be able to read them out in index order and save the sort step. A lot will depend on the details of the query and the tables involved– your saying you “know the problem is with the DISTINCT” really limits the scope of available answers.
How many rows does a distinct query need?
The table in question has 2.5 million rows of data. The DISTINCT is needed for purposes not listed here (because I don’t want back a modified query, but rather just general information about making distinct queries run faster at the DBMS level, if possible).
How to speed up count ( * ) in SQL Server?
You could also try adding page compression to your index. That could make your query faster but it depends on the data in the table. If you’re able to upgrade to SQL Server 2016 then you can create a nonclustered columnstore index on the table. That will make COUNT (*) queries extremely fast with a lower overhead on DML operations.
Is it slow to use subqueries in MySQL?
Subqueries execute every time you evaluate them (in MySQL anyway, not all RDBMSes), i.e. you’re basically running 7 million queries! Using a JOIN, if possible, will reduce this to 1. Even if adding indexing improves performance of those, you’re still running them. Yes, IN with subqueries is slow. Use a join instead.
How to speed up PostgreSQL count ( distinct ) query?
The same plan as for the CTE could probably also be produced by other methods (window functions) If your count (distinct (x)) is significantly slower than count (x) then you can speed up this query by maintaining x value counts in different table, for example table_name_x_counts (x integer not null, x_count int not null), using triggers.
How to speed up the count ( x ) query?
If your count(distinct(x)) is significantly slower than count(x) then you can speed up this query by maintaining x value counts in different table, for example table_name_x_counts (x integer not null, x_count int not null), using triggers.
When to use the SELECT DISTINCT statement 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. SELECT DISTINCT column1, column2,
What to do when SQL query joins multiple tables?
Create an index on these tables or define the joined field as a PRIMARY KEY. As i can see, t1 table is the one which is being joined with all the tables, instead of putting them in a single query with so many joins, you can possibly try a Union of different queries something like this.