Add new column without table lock? - sql

Question

In my project having 23 million records and around 6 fields has been indexed of that table.
Earlier I tested to add delta column for Thinking Sphinx search but it turns in holding the whole database lock for an hour. Afterwards when the file is added and I try to rebuild indexes this is the query that holds the database lock for around 4 hours:
"update user_messages set delta = false where delta = true"
Well for making the server up I created a new database from db dump and promote it as database so server can be turned live.
Now what I am looking is that adding delta column in my table with out table lock is it possible? And once the column delta is added then why is the above query executed when I run the index rebuild command and why does it block the server for so long?
PS.: I am on Heroku and using Postgres with ika db model.

Answer 1

Postgres 11 or later
Since Postgres 11, only volatile default values still require a table rewrite. The manual:
Adding a column with a volatile DEFAULT or changing the type of an existing column will require the entire table and its indexes to be rewritten.
Bold emphasis mine. false is immutable. So just add the column with DEFAULT false. Super fast, job done:
ALTER TABLE tbl ADD column delta boolean DEFAULT false;
Postgres 10 or older, or for volatile DEFAULT
Adding a new column without DEFAULT or DEFAULT NULL will not normally force a table rewrite and is very cheap. Only writing actual values to it creates new rows. But, quoting the manual:
Adding a column with a DEFAULT clause or changing the type of an
existing column will require the entire table and its indexes to be rewritten.
UPDATE in PostgreSQL writes a new version of the row. Your question does not provide all the information, but that probably means writing millions of new rows.
While doing the UPDATE in place, if a major portion of the table is affected and you are free to lock the table exclusively, remove all indexes before doing the mass UPDATE and recreate them afterwards. It's faster this way. Related advice in the manual.
If your data model and available disk space allow for it, CREATE a new table in the background and then, in one transaction: DROP the old table, and RENAME the new one. Related:
Best way to populate a new column in a large table?
While creating the new table in the background: Apply all changes to the same row at once. Repeated updates create new row versions and leave dead tuples behind.
If you cannot remove the original table because of constraints, another fast way is to build a temporary table, TRUNCATE the original one and mass INSERT the new rows - sorted, if that helps performance. All in one transaction. Something like this:
BEGIN
SET temp_buffers = 1000MB; -- or whatever you can spare temporarily
-- write-lock table here to prevent concurrent writes - if needed
LOCK TABLE tbl IN SHARE MODE;
CREATE TEMP TABLE tmp AS
SELECT *, false AS delta
FROM tbl; -- copy existing rows plus new value
-- ORDER BY ??? -- opportune moment to cluster rows
-- DROP all indexes here
TRUNCATE tbl; -- empty table - truncate is super fast
ALTER TABLE tbl ADD column delta boolean DEFAULT FALSE; -- NOT NULL?
INSERT INTO tbl
TABLE tmp; -- insert back surviving rows.
-- recreate all indexes here
COMMIT;

Answer 2

You could add another table with the one column, there won't be any such long locks. Of course there should be another column, a foreign key to the first column.
For the indexes, you could use "CREATE INDEX CONCURRENTLY", it doesn't use too heavy locks on this table http://www.postgresql.org/docs/9.1/static/sql-createindex.html.