SQL 修剪非常慢

Question

We have millions of records in table. We have a column that has white-spaces after the text. We have tried the following:

update schema.table_name 
set column_name = trim(column_name);

It is very slow 20+ hours. We have tried to use the WHERE clause and the RTRIM too however it made no difference. What can be done to speed up this trim?

Answer 1

What is making the query slow is probably not the trim() but rather having to update every row. The trim() would be slow if the column is particularly large.

Assuming that only a few rows need to be updated, then you can filter with a where :

update schema.table_name
    set column_name = trim(column_name)
    where column_name like ' %' or column_name like '% ';

If most rows need to be updated, then reconstructing the table is often the fastest solution:

create table temp_table_name
    select . . ., -- all the other columns
           trim(column_name) as column_name
    from t;

truncate table table_name;  -- BACKUP THE TABLE FIRST!

insert into table_name
    select *
    from temp_table_name;

The exact syntax might vary by database. Also, this is tricky if you have auto-incrementing columns. But the idea is the same.