How to efficiently update postgres using a tuple of the PK and a value?

问题

My SCHEMA is the following and I have ~ 4m existing posts in the DB that I need to update. I am adding an integer which points to a text location.

CREATE TABLE app_post (
id integer NOT NULL,
text_location integer,
title character varying(140)
);

I want to update existing records with a long (1000-5000) list of tuples that represent (id, text_location):

[(1, 123), (2,3), (9, 10)....]

What is the most efficient way to do this?

回答1:

If you are generating the values on the fly using phyton, you could:

Create a buffer containing a single INSERT statement
Start a transaction
Create a temporary table and perform the INSERT statement in your buffer
Perform an UPDATE ... FROM
Commit the transaction, discarding the temporary table.

The UPDATE statement will look like this (assuming there is a table new_values containing those new values you need to update):

UPDATE app_post AS a SET text_location = n.text_location 
    FROM new_values AS n WHERE a.id = n.id

Don't forget to define the columns id as PRIMARY KEY or create an index on them.

EDIT : Since you are experiencing very slow performance, another workaround could be to recreate the whole table. The following idea assumes you don't have any FOREIGN KEY constraint applied to app_post table, as you have shown in your initial post.

-- Begin the Transaction
BEGIN;

-- Create a temporary table to hold the new values
CREATE TEMPORARY TABLE temp_update_values (
    id integer PRIMARY KEY,
    text_location integer
) ON COMMIT DROP;

-- Populate it
INSERT INTO temp_update_values (id, text_location) VALUES (1, 123), (2, 456) /* ... #5000 total */ ;

-- Create a temporary table merging the existing "app_post" and "temp_update_values"
CREATE TEMPORARY TABLE temp_new_app_post ON COMMIT DROP AS 
    SELECT a.id, COALESCE(n.text_location, a.text_location) AS text_location, a.title 
    FROM app_post AS a LEFT JOIN temp_update_values AS n ON a.id = n.id;

-- Empty the existing "app_post"
TRUNCATE TABLE app_post;

-- Repopulate "app_post" table 
INSERT INTO app_post (id, text_location, title) 
    SELECT id, text_location, title FROM temp_new_app_post;

-- Commit the Transaction
COMMIT;

If there are any FOREIGN KEY constraint, you should take care of them, dropping them before TRUNCATING the app_post table, and re-creating them after it's been repopulated.

来源：https://stackoverflow.com/questions/18767115/how-to-efficiently-update-postgres-using-a-tuple-of-the-pk-and-a-value

标签

sql

postgresql

tuples