I've a table that stores hierarchical information in a ltree field for fast
access, indexed with a GiST index.
Interesting Problem with ltree/GiST index: sometimes, the index will get
corrupt, as seen by the examples below:
WRONG
db=# SELECT path from strukturelemente where path ~
'142.2330445.2330526.5358672.5358675.5358752.*';
path
------
(0 rows)
RIGHT
db=# SELECT path from strukturelemente where path ~
'*.2330445.2330526.5358672.5358675.5358752.*';
path
---------------------------------------------
142.2330445.2330526.5358672.5358675.5358752
(1 row)
db=# SELECT path from strukturelemente where path ~ '*.5358752.*';
path
---------------------------------------------
142.2330445.2330526.5358672.5358675.5358752
(1 row)
db=# EXPLAIN ANALYZE SELECT path from strukturelemente where path ~
'*.5358752.*';
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on strukturelemente (cost=2.05..27.99 rows=14 width=45)
(actual time=4.617..4.618 rows=1 loops=1)
Recheck Cond: (path ~ '*.5358752.*'::lquery)
-> Bitmap Index Scan on str_uk4 (cost=0.00..2.05 rows=14 width=0) (actual
time=4.604..4.604 rows=1 loops=1)
Index Cond: (path ~ '*.5358752.*'::lquery)
Total runtime: 4.690 ms
(5 rows)
db=# EXPLAIN ANALYZE SELECT path from strukturelemente where path ~
'142.2330445.2330526.5358672.5358675.5358752.*';
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on strukturelemente (cost=2.05..27.99 rows=14 width=45)
(actual time=0.030..0.030 rows=0 loops=1)
Recheck Cond: (path ~
'142.2330445.2330526.5358672.5358675.5358752.*'::lquery)
-> Bitmap Index Scan on str_uk4 (cost=0.00..2.05 rows=14 width=0) (actual
time=0.027..0.027 rows=0 loops=1)
Index Cond: (path ~
'142.2330445.2330526.5358672.5358675.5358752.*'::lquery)
Total runtime: 0.081 ms
(5 rows)
db=# SELECT path from strukturelemente where path ~
'142.2330445.2330526.5358672.5358675.*.5358752.*';
path
------
(0 rows)
When doing sequential scans (set enable_indexscan to off and set
enable_bitmapscan to off) everything works as expected.
db=# SELECT path from strukturelemente where path ~
'142.2330445.2330526.5358672.*.5358675.5358752.*';
path
---------------------------------------------
142.2330445.2330526.5358672.5358675.5358752
(1 row)
After an update (and turning index access on), everything will work fine again:
UPDATE strukturelemente set id=id where id=5358752;
I get this:
db=# SELECT path from strukturelemente where path ~
'142.2330445.2330526.5358672.5358675.5358752.*';
path
---------------------------------------------
142.2330445.2330526.5358672.5358675.5358752
(1 row)
db=# SELECT path from strukturelemente where path ~
'142.2330445.2330526.5358672.5358675.*';
path
---------------------------------------------
142.2330445.2330526.5358672.5358675.5358752
(1 row)
Reindexing the index also fixes the problem, but regular reindexing cannot be a
solution, since there is still a timeframe where wrong results are returned.
I've made a copy of the index-file when it was broken, after the update and
after reindexing. Is there a tool for getting a human-readable dump of the
index?
Using PostgreSQL 8.1.4 with ltree version from CVS (since there's another bug
fixed in CVS). Problem occurs on a 2 node cluster of 2 Quad-CPU system (Xeon, 2
physical CPU's, 2 hyperthreading), one node running the database, the other one
the webserver.
Any ideas?
Best regards,
Mario Weilguni
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