Is a value that does not change calculated for each row? Consider an SQL query with the following WHERE clause:

WHERE expires &gt;= DATE_FORMAT(NOW(), '%Y%m%d')


Will the result of DATE_FORMAT() be recalculated for each row, or will it be calculated once and used for each row? If the latter, I can calculate the value in Python and then inject it into the SQL sent to MySQL.

Due to application restraints, the value is actually a LONGTEXT column and this question is a simplification of the problem. The data must be stored in this column type due to the EAV nature of the table.

This question pertains to MySQL 5.7, but if other database management systems, particularly SQLite or Postgres have different behaviour then I would appreciate knowing that as well. Thank you.
As long as it is run as a single transaction, the value will be re-used on each line, not recalculated.  I'm not 100% sure about Postgres or SQL Lite but I'm fairly confident (98%) they will work the same.  

In the past when I was more confused about this question, I would pre-fetch the date value to a variable and then reuse the variable in the where clause.  Now I realize that was pointless unless I have more than one place in a longer set of statements where I would like to use the value.  

As a test, move the calculated date value to the select clause and execute against a large table that takes more than a few seconds to return.  You will notice that the first line will have the same date/time value as the last record.