How to improve the select query performance for to find the count in a table in oracle

>>select count(*) from schema.tablename;

(1) do you have index on this table? if you have index, the query will likely to access only index to get better performance instead of full table scan on table data.

(2) This query usually will use FTS ( full table scan). FTS will scan till HWM. In order to let FTS scan less blocks, you could do is to
      1.  reorgnize your table to lower blocks used thus HWM.
      2. You could use "parallel query" to improve the query performance.


acton      
     
       

Why do you want to do a "select count(*) from schema.tablename" on a large table?  Do you need an exact count that is up-to-date?  Or, could you analyze the table at regular intervals (like at night or on the weekend) and just do:
select num_rows from all_tables where owner = '[schema]' and table_name = '[table_name]'

or:
select table_name, num_rows from all_tables
where owner = '[schema]'

to markgeer:

num_rows  is popluated only after you do statistics analysis. Also,  It might not be up to date.

Therefore I don't think you can use it to substitue for  "select count(*) ..."

To actonwang:

Did you notice that I only mentioned using num_rows after saying: "...analyze the table at regular intervals...".

to markegeer,

     >>it might not be up to date.

     did you see that?

Right, the value for "num_rows" will only be as up-to-date as the last time that the table was analyzed.

(Also, I had asked the questioner if he/she needs an up-to-date count and we are still waiting for an answer to that.)

heahea :)

If you have a reporting database (a data warehouse) then a bit-mapped index may be a good option.  *DO NOT* create a bit-mapped index though on a table used in a transaction-processing system by multiple users!