如何使用python和postgres将表的奇数行插入另一个表

Question

I have a python program in which I want to read the odd rows from one table and insert them into another table. How can I achieve this?

For example, the first table has 5 rows in total, and I want to insert the first, third, and fifth rows into another table.

Note that the table may contains millions of rows, so the performance is very important.

Answer 1

I found a few methods here . Here's two of them transcribed to psycopg2 .

If you have a sequential primary key, you can just use mod on it:

database_cursor.execute('SELECT * FROM table WHERE mod(primary_key_column, 2) = 1')

Otherwise, you can use a subquery to get the row number and use mod :

database_cursor.execute('''SELECT col1, col2, col3
                             FROM (SELECT row_number() OVER () as rnum, col1, col2, col3
                                     FROM table)
                           WHERE mod(rnum, 2) = 1''')

Answer 2

If you have an id-type column that is guaranteed to increment by 1 upon every insert (kinda like an auto-increment index), you could always mod that to select the row. However, this would break when you begin to delete rows from the table you are selecting from.

A more complicated solution would be to use postgresql's row_number() function. The following assumes you have an id column that can be used to sort the rows in the desired order:

select r.* 
from (select *,row_number() over(order by id) as row 
      from <tablename>
) r
where r.row % 2 = 0

Note: regardless of how you do it, the performance will NEVER really be efficient as you necessarily have to do a full table scan, and selecting all columns on a table with millions of records using a full table scan is going to be slow.