Flip flopping data tables in Postgres - postgresql

Question

I have a table of several million records which I am running a query against and inserting the results into another table which clients will query. This process takes about 20 seconds.
How can I run this query, building this new table without impacting any of the clients that might be running queries against the target table?
For instance. I'm running
BEGIN;
DROP TABLE target_table;
SELECT blah, blahX, blahY
INTO target_table
FROM source_table
GROUP BY blahX, blahY
COMMIT;
Which is then blocking queries to:
SELECT SUM(blah)
FROM target_table
WHERE blahX > x
In the days of working with some SQL Server DBA's I recall them creating temporary tables, and then flipping these in over the current table. Is this doable/practical in Postgres?

Answer 1

What you want here is to minimize the lock time, which of course if you include a query (that takes a while) in your transaction is not going to work.
In this case, I assume you're in fact refreshing that 'target_table' which contains the positions of the "blah" objects when you run your script is that correct ?
BEGIN;
CREATE TEMP TABLE temptable AS
SELECT blah, blahX, blahY
FROM source_table
GROUP BY blahX, blahY
COMMIT;
BEGIN;
TRUNCATE TABLE target_table
INSERT INTO target_table(blah,blahX,blahY)
SELECT blah,blahX,blahY FROM temptable;
DROP TABLE temptable;
COMMIT;
As mentioned in the comments, it will be faster to drop the index's before truncating and create them anew just after loading the data to avoid the unneeded index changes.
For the full details of what is and is not possible with postgreSQL in that regard :
http://postgresql.1045698.n5.nabble.com/ALTER-TABLE-REPLACE-WITH-td3305036i40.html

Answer 2

There's ALTER TABLE ... RENAME TO ...:
ALTER TABLE name
RENAME TO new_name
Perhaps you could select into an intermediate table and then drop target_table and rename the intermediate table to target_table.
I have no idea how this would interact with any queries that may be running against target_table when you try to do the rename.

Answer 3

You can create a table, drop a table, and rename a table in every version of SQL I've ever used.
BEGIN;
SELECT blah, blahX, blahY
INTO new_table
FROM source_table
GROUP BY blahX, blahY;
DROP TABLE target_table;
ALTER TABLE new_table RENAME TO target_table;
COMMIT;
I'm not sure off the top of my head whether you could use a temporary table for this in PostgreSQL. PostgreSQL creates temp tables in a special schema; you don't get to pick the schema. But you might be able to create it as a temporary table, drop the existing table, and move it with SET SCHEMA.
At some point, any of these will require a table lock. (Duh.) You might be able to speed things up a lot by putting the swappable table on a SSD.