为PostgreSQL中的数千条记录分配顺序位置

Question

Using Ruby on Rails —but hopefully this can be solved via PostgreSQL directly, I'm trying to move 100,000 items from one list to another, while assigning each of them a sequential position:

List A (where position don't matter):

- item 1, position: nil
- item 2, position: nil
- item 3, position: nil

List B (where positions do matter):

- item 4, position: 1

And as a result, I want:

List B:

- item 4, position: 1
- item 1, position: 2
- item 2, position: 3
- item 3, position: 4

Various strategies I've thought about so far:

1. one SQL query per item, but this gets slow when moving 100k items :(
2. dynamically creating one SQL sequence per list, but this feels like the wrong approach as I'm expecting 1000 new lists per day
3. one SQL query for the whole list where the position is set using the max position, which would probably rely on a SQL loop?
4. extracting the problem to a different tool (Redis might be better suited for the end solution, but I'd like to explore solutions with available tools first)

Answer 1

I am not entirely sure what are you trying to do here, but I suspect that sequences would be the best option here (but not dynamically created ones, that almost always a bad idea). If you really don't want to use sequences, you could also use window functions and specificaly the row_number windows function.

Here's a complete example. Suppose that you have the following tables with some sample data:

CREATE TABLE ListA (
    item integer NOT NULL,
    position integer
);

CREATE TABLE ListB ( item integer NOT NULL, position integer );

INSERT INTO ListA(item) VALUES (1),(2),(3);
INSERT INTO ListB(item,position) VALUES (4,1);

Then the following query returns all elements of ListA but with a new position assigned to them (starting from 1):

SELECT item,row_number() OVER (ORDER BY item)
FROM ListA;

If you would like to start the position numbers from where ListB stops, you can do it like that

SELECT item,row_number() OVER (ORDER BY item)+(SELECT max(position) FROM ListB)
FROM ListA;

Finally, to add those rows to ListB , you can do that like so:

INSERT INTO ListB(item,position)
SELECT item,row_number() OVER (ORDER BY item)+(SELECT max(position) FROM ListB)
FROM ListA;