Number of Rows in a Table

SQL count rows in a table

Yes, SELECT COUNT(*) FROM TableName

Fastest way to count exact number of rows in a very large table?

Simple answer:

• Database vendor independent solution = use the standard = COUNT(*)
• There are approximate SQL Server solutions but don't use COUNT(*) = out of scope

Notes:

COUNT(1) = COUNT(*) = COUNT(PrimaryKey) just in case

Edit:

SQL Server example (1.4 billion rows, 12 columns)

SELECT COUNT(*) FROM MyBigtable WITH (NOLOCK)
-- NOLOCK here is for me only to let me test for this answer: no more, no less

1 runs, 5:46 minutes, count = 1,401,659,700

--Note, sp_spaceused uses this DMV
SELECT
   Total_Rows= SUM(st.row_count)
FROM
   sys.dm_db_partition_stats st
WHERE
    object_name(object_id) = 'MyBigtable' AND (index_id < 2)

2 runs, both under 1 second, count = 1,401,659,670

The second one has less rows = wrong. Would be the same or more depending on writes (deletes are done out of hours here)

In SQL Server, how to find number of rows in a table without doing a count?

Sql have a function for that

sp_spaceused <Tablename>;

Fast way to discover the row count of a table in PostgreSQL

Counting rows in big tables is known to be slow in PostgreSQL. The MVCC model requires a full count of live rows for a precise number. There are workarounds to speed this up dramatically if the count does not have to be exact like it seems to be in your case.

(Remember that even an "exact" count is potentially dead on arrival under concurrent write load.)

Exact count

Slow for big tables.

With concurrent write operations, it may be outdated the moment you get it.

SELECT count(*) AS exact_count FROM myschema.mytable;

Estimate

Extremely fast:

SELECT reltuples AS estimate FROM pg_class where relname = 'mytable';

Typically, the estimate is very close. How close, depends on whether ANALYZE or VACUUM are run enough - where "enough" is defined by the level of write activity to your table.

Safer estimate

The above ignores the possibility of multiple tables with the same name in one database - in different schemas. To account for that:

SELECT c.reltuples::bigint AS estimate
FROM   pg_class c
JOIN   pg_namespace n ON n.oid = c.relnamespace
WHERE  c.relname = 'mytable'
AND    n.nspname = 'myschema';

The cast to bigint formats the real number nicely, especially for big counts.

Better estimate

SELECT reltuples::bigint AS estimate
FROM   pg_class
WHERE  oid = 'myschema.mytable'::regclass;

Faster, simpler, safer, more elegant. See the manual on Object Identifier Types.

Replace 'myschema.mytable'::regclass with to_regclass('myschema.mytable') in Postgres 9.4+ to get nothing instead of an exception for invalid table names. See:

• How to check if a table exists in a given schema

Better estimate yet (for very little added cost)

This does not work for partitioned tables because relpages is always -1 for the parent table (while reltuples contains an actual estimate covering all partitions) - tested in Postgres 14.

You have to add up estimates for all partitions instead.

We can do what the Postgres planner does. Quoting the Row Estimation Examples in the manual:

These numbers are current as of the last VACUUM or ANALYZE on the
table. The planner then fetches the actual current number of pages in
the table (this is a cheap operation, not requiring a table scan). If
that is different from relpages then reltuples is scaled
accordingly to arrive at a current number-of-rows estimate.

Postgres uses estimate_rel_size defined in src/backend/utils/adt/plancat.c, which also covers the corner case of no data in pg_class because the relation was never vacuumed. We can do something similar in SQL:

Minimal form

SELECT (reltuples / relpages * (pg_relation_size(oid) / 8192))::bigint
FROM   pg_class
WHERE  oid = 'mytable'::regclass;  -- your table here

Safe and explicit

SELECT (CASE WHEN c.reltuples < 0 THEN NULL       -- never vacuumed
             WHEN c.relpages = 0 THEN float8 '0'  -- empty table
             ELSE c.reltuples / c.relpages END
     * (pg_catalog.pg_relation_size(c.oid)
      / pg_catalog.current_setting('block_size')::int)
       )::bigint
FROM   pg_catalog.pg_class c
WHERE  c.oid = 'myschema.mytable'::regclass;      -- schema-qualified table here

Doesn't break with empty tables and tables that have never seen VACUUM or ANALYZE. The manual on pg_class:

If the table has never yet been vacuumed or analyzed, reltuples contains -1 indicating that the row count is unknown.

If this query returns NULL, run ANALYZE or VACUUM for the table and repeat. (Alternatively, you could estimate row width based on column types like Postgres does, but that's tedious and error-prone.)

If this query returns 0, the table seems to be empty. But I would ANALYZE to make sure. (And maybe check your autovacuum settings.)

Typically, block_size is 8192. current_setting('block_size')::int covers rare exceptions.

Table and schema qualifications make it immune to any search_path and scope.

Either way, the query consistently takes < 0.1 ms for me.

More Web resources:

• The Postgres Wiki FAQ
• The Postgres wiki pages for count estimates and count(*) performance

TABLESAMPLE SYSTEM (n) in Postgres 9.5+

SELECT 100 * count(*) AS estimate FROM mytable TABLESAMPLE SYSTEM (1);

Like @a_horse commented, the added clause for the SELECT command can be useful if statistics in pg_class are not current enough for some reason. For example:

• No autovacuum running.
• Immediately after a large INSERT / UPDATE / DELETE.
• TEMPORARY tables (which are not covered by autovacuum).

This only looks at a random n % (1 in the example) selection of blocks and counts rows in it. A bigger sample increases the cost and reduces the error, your pick. Accuracy depends on more factors:

• Distribution of row size. If a given block happens to hold wider than usual rows, the count is lower than usual etc.
• Dead tuples or a FILLFACTOR occupy space per block. If unevenly distributed across the table, the estimate may be off.
• General rounding errors.

Typically, the estimate from pg_class will be faster and more accurate.

Answer to actual question

First, I need to know the number of rows in that table, if the total
count is greater than some predefined constant,

And whether it ...

... is possible at the moment the count pass my constant value, it will
stop the counting (and not wait to finish the counting to inform the
row count is greater).

Yes. You can use a subquery with LIMIT:

SELECT count(*) FROM (SELECT 1 FROM token LIMIT 500000) t;

Postgres actually stops counting beyond the given limit, you get an exact and current count for up to n rows (500000 in the example), and n otherwise. Not nearly as fast as the estimate in pg_class, though.

SQL to find the number of rows per table in a database

You can use this query against the system catalog views:

SELECT 
    t.NAME AS TableName,
    p.rows AS RowCounts
FROM 
    sys.tables t
INNER JOIN      
    sys.indexes i ON t.OBJECT_ID = i.object_id
INNER JOIN 
    sys.partitions p ON i.object_id = p.OBJECT_ID AND i.index_id = p.index_id
WHERE 
    t.NAME NOT LIKE 'dt%' 
    AND t.is_ms_shipped = 0
    AND i.OBJECT_ID > 255 
GROUP BY 
    t.Name, p.Rows
ORDER BY 
    t.Name

to get the desired result - all tables in the current database, and the number of rows in each.

count number of rows in table using php

 <?php
  $mysqli = new mysqli("hostname", "dbusername", "dbpassword", "dbname");

   /* check connection */
   if (mysqli_connect_errno()) {
    printf("Connect failed: %s\n", mysqli_connect_error());
     exit();
   }

  if ($result = $mysqli->query("SELECT count(*) cc FROM tablename")) {

     /* fetch the first row as result */
     $row = $result->fetch_assoc();

    printf("Result set has %d rows.\n", $row['cc']);

   /* close result set */
    $result->close();
 }

 /* close connection */
 $mysqli->close();
?>

In fact it's a common question you can find the answer anywhere.

Like, http://php.net/manual/en/mysqli-result.num-rows.php

You could separate this problem in to two

1. I wanna know how to connect to mysql.
2. I wanna know how to write that sql instruction.

Get the number of rows in a HTML table

Try:

var count = $('#gvPerformanceResult tr').length;

How to count the number of rows in a table using Selenium Java

Try below solution :

List<WebElement> rowCount = driver.findElements(By.xpath("//tbody[@id='MyTableId']/tbody/tr"));
System.out.println("Num rows: " + rowCount .size());

Extracting number of rows from a sql table

...but the resulting request that is select count(*) from (select * from employees)) is not a correct one...

You are using your query as a "table expression" but you forgot to assign an alias to it. Try:

select count(*) from (select * from employees)) x

See the x at the end?

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