If you are running the queries in a live database you may need to handle concurrent write requests. Handling Concurrent WritesĮven with the aforementioned optimizations, recreating your table in PostgreSQL is a slow operation. SELECT * FROM tmp_tbl - insert the rows back - recreate all indexes if needed 3. ) - DROP indexes from tbl if needed TRUNCATE tbl - alter tbl and add new columns if needed INSERT INTO tbl SET temp_buffers = 3000MB - change this value accordingly - create and populate the temporary table Using RAM instead of the disk to store the temporary table will obviously improve the performance: If your table can fit in memory you should increase the temp_buffers during this transaction. We will demonstrate this in the next section. This method has some advantages when you have pending write requests. If you can’t drop the original table because you don’t want to recreate views or there are other unique constraints, then you can use a temporary table to hold the new values, truncate the old table and rewrite the data there. ) CREATE INDEX - add your constraints and indexes to new_tbl DROP TABLE tbl ALTER TABLE tbl_new RENAME TO tbl 3. use your new logic here to insert the updated data Here is a script with the base structure for this operation: CREATE TABLE new_tbl Then, the easiest way to perform the update is to insert the data into a new table and rename it afterwards. If you can safely drop the existing table and if there is enough disk space. The fastest way to update a large table is to create a new one. It may also require more complex application logic during the migration. It is a very slow process because in place updates are costly. The main problem with this approach is the performance. When adding a new column, you can temporarily set it as nullable then gradually fill it with new values. This maximizes your table availability since you only need to keep locks for a short period of time. If you can segment your data using, for example, sequential IDs, you can update rows incrementally in batches. With this in mind, let’s look at a few strategies that you can use to effectively update a large number of rows in your table in PostgreSQL: 1. Strategies To Update Tables In PostgresSQL Ex: conversion from VARCHAR(32) to VARCHAR(64).
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