SQL getting row count optimization [duplicate]

Question

Possible Duplicate:
In SQL, what's the difference between count(column) and count()?
Count() vs Count(1)

I have big tables which keep long texts for example email content or news. And my question is there any difference for performance for counting table rows :

SELECT COUNT(*) FROM table_name

SELECT COUNT(t.id) FROM table_name as t

Which one is better? or the latter one will be optimized by query optimizer? Is there any documentation regarding this?

Answer 1

They are different.

COUNT(*) will retrieve all values (even the NULL values count). COUNT(t.id) doesn't count the NULL values.

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In terms of performance, they are the same (the query optimizer is smart).

Answer 2

Query plan looks the same, but in testing, using * is faster (slightly).

Nulls are not taking into account when specifying a column name though.