How do I speed up counting rows in a PostgreSQL table? - sql

Question

We need to count the number of rows in a PostgreSQL table. In our case, no conditions need to be met, and it would be perfectly acceptable to get a row estimate if that significantly improved query speed.
Basically, we want select count(id) from <table> to run as fast as possible, even if that implies not getting exact results.

Answer 1

For a very quick estimate:
SELECT reltuples FROM pg_class WHERE relname = 'my_table';
There are several caveats, though. For one, relname is not necessarily unique in pg_class. There can be multiple tables with the same relname in multiple schemas of the database. To be unambiguous:
SELECT reltuples::bigint FROM pg_class WHERE oid = 'my_schema.my_table'::regclass;
If you do not schema-qualify the table name, a cast to regclass observes the current search_path to pick the best match. And if the table does not exist (or cannot be seen) in any of the schemas in the search_path you get an error message. See Object Identifier Types in the manual.
The cast to bigint formats the real number nicely, especially for big counts.
Also, reltuples can be more or less out of date. There are ways to make up for this to some extent. See this later answer with new and improved options:
Fast way to discover the row count of a table in PostgreSQL
And a query on pg_stat_user_tables is many times slower (though still much faster than full count), as that's a view on a couple of tables.

Answer 2

Count is slow for big tables, so you can get a close estimate this way:
SELECT reltuples::bigint AS estimate
FROM pg_class
WHERE relname='tableName';
and its extremely fast, results are not float, but still a close estimate.
reltuples is a column from pg_class table, it holds data about "number of rows in the table. This is only an estimate used by the planner. It is updated by VACUUM, ANALYZE, and a few DDL commands such as CREATE INDEX" (manual)
The catalog pg_class catalogs tables and most everything else that has columns or is otherwise similar to a table. This includes indexes (but see also pg_index), sequences, views, composite types, and some kinds of special relation (manual)
"Why is "SELECT count(*) FROM bigtable;" slow?" : http://wiki.postgresql.org/wiki/FAQ#Why_is_.22SELECT_count.28.2A.29_FROM_bigtable.3B.22_slow.3F

Answer 3

Aside from running COUNT() on an indexed field (which hopefully 'id' is) - the next best thing would be to actually cache the row count in some table using a trigger on INSERT. Naturally, you'll be checking the cache instead.
For an approximation you can try this (from https://wiki.postgresql.org/wiki/Count_estimate):
select reltuples from pg_class where relname='tablename';

Answer 4

You can get an estimate from the system table "pg_stat_user_tables".
select schemaname, relname, n_live_tup
from pg_stat_user_tables
where schemaname = 'your_schema_name'
and relname = 'your_table_name';

Answer 5

You can ask for the exact value of the count in the table by simply using trigger AFTER INSERT OR DELETE
Something like this
CREATE TABLE tcounter(id serial primary key,table_schema text, table_name text, count serial);
insert into tcounter(table_schema, table_name,count) select 'my_schema', 'my_table', count(*) from my_schema.my_table;
and use trigger
CREATE OR REPLACE FUNCTION ex_count()
RETURNS trigger AS
$BODY$
BEGIN
IF (TG_OP='INSERT') THEN
UPDATE tcounter set count = count + 1 where table_schema = TG_TABLE_SCHEMA::TEXT and table_name = TG_TABLE_NAME::TEXT;
ELSIF (TG_OP='DELETE') THEN
UPDATE tcounter set count = count - 1 where table_schema = TG_TABLE_SCHEMA::TEXT and table_name = TG_TABLE_NAME::TEXT;
END IF;
RETURN NEW;
END$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
CREATE TRIGGER tg_counter AFTER INSERT OR DELETE
ON my_schema.my_table FOR EACH ROW EXECUTE PROCEDURE ex_count();
And ask for count
select * from tcounter where table_schema = 'my_schema' and table_name = 'my_table'
it means you select count(*) once for initialize first record

Answer 6

If your database is small, you can get an estimate of all your tables like #mike-sherrill-cat-recall suggested. This command will list all the tables though.
SELECT schemaname,relname,n_live_tup
FROM pg_stat_user_tables
ORDER BY n_live_tup DESC;
Output would be something like this:
schemaname | relname | n_live_tup
------------+--------------------+------------
public | items | 21806
public | tags | 11213
public | sessions | 3269
public | users | 266
public | shops | 259
public | quantities | 34
public | schema_migrations | 30
public | locations | 8
(8 rows)