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When you run something similar to:
UPDATE table SET datetime = NOW(); on a table with 1 000 000 000 records and the query takes 10 seconds to run, will all the rows have the exact same time (minutes and seconds) or will they have different times? In other words, will the time be when the query started or when each row is updated?
I'm running MySQL, but I'm thinking this applies to all dbs.
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To update an entire row in MySQL, use UPDATE command. You need to know the primary key column. The syntax is as follows to update an entire row.
MySQL UPDATE command can be used to update multiple columns by specifying a comma separated list of column_name = new_value. Where column_name is the name of the column to be updated and new_value is the new value with which the column will be updated.
Syntax – Update value to Current Timestamp ALTER TABLE table_name updates table schema. CHANGE column_name updates the column to. column_name TIMESTAMP NOT NULL defines the column as of datatype TIMESTAMP. DEFAULT CURRENT_TIMESTAMP sets the default value of the column to CURRENT_TIMESTAMP.
The CURRENT_TIMESTAMP() function returns the current date and time. Note: The date and time is returned as "YYYY-MM-DD HH-MM-SS" (string) or as YYYYMMDDHHMMSS. uuuuuu (numeric).
http://dev.mysql.com/doc/refman/5.0/en/date-and-time-functions.html#function_now
"NOW() returns a constant time that indicates the time at which the statement began to execute. (Within a stored routine or trigger, NOW() returns the time at which the routine or triggering statement began to execute.) This differs from the behavior for SYSDATE(), which returns the exact time at which it executes as of MySQL 5.0.13. "
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