Re: BUG #19686: Rolling back SET TABLESPACE - Mailing list pgsql-hackers

This patchset addresses the issue reported on the pgsql-bugs [1]

The root cause is that after the relation files are copied to a new tablespace queries

update the index in place but the heap is updated only in the tablespace copy. If

the transaction is rolled back, the index and the heap becomes inconsistent.

First I tried to fix the crash, easy for btree, manageable for hash, but for GiST that

would not be feasible, AFAIK would have to perform array searches possibly over

multiple pages. Later thinking about this I noticed something I didn't realise on my

first read.

Even without inserting duplicates, and no crashes, it can produce incorrect results.

SET enable_seqscan = off;
SET enable_bitmapscan = off;
SET allow_in_place_tablespaces = true;
CREATE TABLESPACE ts LOCATION '';
CREATE TABLE t(a int);
CREATE INDEX ON t(a);
BEGIN;
ALTER TABLE t SET TABLESPACE ts;
INSERT INTO t VALUES (0); -- this adds (0, 1) | 0 to the index in the ts copy
ROLLBACK;
INSERT INTO t VALUES (41); -- this adds (0, 1) | 41 in the default tablespace
SELECT ctid, a FROM t WHERE a = 0;
 ctid  | a  
-------+----
 (0,1) | 41
(1 row)

So, I decided to fix the root cause: modifying a non-durable copy of the file.

I thought it would be way harder, but the code was architected well enough

that I could save a list of deferred copies, and keep modifying the the table

in place. If the transaction is rolled back all the tuples in the index will have

its (possibly dead) in the heap, effectively reserving those TID, this prevents

both the insertion of duplicates, and the resuscitation of dead tuples by

later changes.

[1] https://www.postgresql.org/message-id/19686-30f4da834585129f%40postgresql.org