是否可以在MySQL表的顶部插入新行？

Question

All rows in MySQL tables are being inserted like this:

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2
3

Is there any way how to insert new row at a top of table so that table looks like this?

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1

Yes, yes, I know "order by" but let me explain the problem. I have a dating website and users can search profiles by sex, age, city, etc. There are more than 20 search criteria and it's not possible to create indexes for each possible combination. So, if I use "order by", the search usually ends with "using temporary, using filesort" and this causes a very high server load. If I remove "order by" oldest profiles are shown as first and users have to go to the last page to see the new profiles. That's very bad because first pages of search results always look the same and users have a feeling that there are no new profiles. That's why I asked this question. If it's not possible to insert last row at top of table, can you suggest anything else?

Answer 1

The order in which the results are returned when there's no ORDER BY clause depends on the RDBM. In the case of MySQL, or at least most engines, if you don't explicitly specify the order it will be ascending, from oldest to new entries. Where the row is located "physically" doesn't matter. I'm not sure if all mysql engines work that way though. Ie, in PostgreSQL the "default" order shows the most recently updated rows first. This might be the way some of the MySQL engines work too.

Anyway, the point is - if you want the results ordered - always specify sort order, don't just depend on something default that seems to work. In you case you want something trivial - you want the users in descending order, so just use:

SELECT * FROM users ORDER BY id DESC

Answer 2

I think you just need to make sure that if you always need to show the latest data first, all of your indexes need to specify the date/time field first, and all of your queries order by that field first.

If ORDER BY is slowing everything down then you need to optimise your queries or your database structure, i would say.

Answer 3

I know that a lot of time has passed since the above question was asked. But I have something to add to the comments:

I'm using MySQL version: 5.7.18-0ubuntu0.16.04.1

When no ORDER BY clause is used with SELECT it is noticeable that records are displayed, regardless of the order in which they are added, in the table's Prime Key sequence.

Answer 4

Maybe if you add the id 'by hand', and give it a negative value, but i (and probably nobody) would recommend you to do that:

1. Regular insert, eg

   insert into t values (...);

2. Update with set, eg

   update t set id = -id where id = last_insert_id();

Answer 5

Normally you specify a auto_incrementing primary key.

However, you can just specify the primary key like so:

CREATE TABLE table1 (
  id signed integer primary key default 1, <<-- no auto_increment, but has a default value
  other fields .....

Now add a BEFORE INSERT trigger that changes the primary key.

DELIMITER $$

CREATE TRIGGER ai_table1_each BEFORE INSERT ON table1 FOR EACH ROW
BEGIN
  DECLARE new_id INTEGER;
  SELECT COALESCE(MIN(id), 0) -1 INTO new_id FROM table1;
  SET NEW.id = new_id;
END $$

DELIMITER ; 

Now your id will start at -1 and run down from there.
The insert trigger will make sure no concurrency problems occur.