[HACKERS] Custom index structure and strange count problem
Hi *,
during the last few months I've been building a new index structure as part of
a research project.
Everything seems to work properly, however I have some strange issues with the
count sql command.
I introduced some custom structures (mainly document and hybrid_query) with
which my index access method is supposed to work.
There is an operator which is supposed to use my index structure (what
also works properly).
The function that maps to the operator is called hybrid_index_query, which
I use to compare my results given from the index with the real results that are
supposed to appear in the final result set.
Having described the outer circumstances (in a very short way), I will now show
the strange stuff that happens:
test=# select id from documents where hybrid_index_query(to_document(words,
points), row('radio pleas news'::tsvector, '[(-90,-180),(90, 180)]')) order by
id;
id
--
2137
2151
2168
2207
2208
2209
2210
2211
2266
2296
(10 rows)
This query takes a sequential scan and works properly (returning 10 rows).
test=# select id from documents where to_document(words, points) row('pleas
radio news'::tsvector, '[(-90,-180),(90,180)]') order by id;
id
--
2137
2151
2168
2207
2208
2209
2210
2211
2266
2296
(10 rows)
This query uses my index structure and returns the same result as in the
sequential scan above.
Until here, everything seems to work fine. However, if I issue the same queries
using the count aggregate function in SQL, there are some odd results:
test=# select count(*) from documents where
hybrid_index_query(to_document(words, points), row('radio pleas
news'::tsvector, '[(-90,-180),(90, 180)]'));
count
---
10
(1 row)
Using the sequential scan, still, everything seems fine.
However, if I now do the index scan (my function will be called 11 times,
returning false at the end), I get the following result:
test=# select count(*) from documents where to_document(words, points)
row('pleas radio news'::tsvector, '[(-90,-180),(90,180)]');
count
---
7
(1 row)
This seems strange, because the same query returned 10 rows (when I didn't use
the aggregate). If I issue queries that count the id column, I receive the
following:
test=# select count(id) from documents where
hybrid_index_query(to_document(words, points), row('radio pleas
news'::tsvector, '[(-90,-180),(90, 180)]'));
count
---
10
(1 row)
test=# select count(id) from documents where to_document(words, points)
row('pleas radio news'::tsvector, '[(-90,-180),(90,180)]');
count
---
10
(1 row)
These two queries do again return the same results.
Thus, I don't know, what's wrong here, does anybody know about that behaviour,
or is it my fault that the results are wrong, somehow?
Thanks in advance
Carsten Kropf
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Re: [HACKERS] Custom index structure and strange count problem
On Wed, Jun 9, 2010 at 4:35 AM, Carsten Kropf ckro...@fh-hof.de wrote:
Hi *,
during the last few months I've been building a new index structure as part
of a research project.
Everything seems to work properly, however I have some strange issues with
the count sql command.
I introduced some custom structures (mainly document and hybrid_query) with
which my index access method is supposed to work.
There is an operator which is supposed to use my index structure (what
also works properly).
The function that maps to the operator is called hybrid_index_query,
which I use to compare my results given from the index with the real results
that are supposed to appear in the final result set.
Having described the outer circumstances (in a very short way), I will now
show the strange stuff that happens:
test=# select id from documents where hybrid_index_query(to_document(words,
points), row('radio pleas news'::tsvector, '[(-90,-180),(90, 180)]')) order
by id;
id
--
2137
2151
2168
2207
2208
2209
2210
2211
2266
2296
(10 rows)
This query takes a sequential scan and works properly (returning 10 rows).
test=# select id from documents where to_document(words, points)
row('pleas radio news'::tsvector, '[(-90,-180),(90,180)]') order by id;
id
--
2137
2151
2168
2207
2208
2209
2210
2211
2266
2296
(10 rows)
This query uses my index structure and returns the same result as in the
sequential scan above.
Until here, everything seems to work fine. However, if I issue the same
queries using the count aggregate function in SQL, there are some odd results:
test=# select count(*) from documents where
hybrid_index_query(to_document(words, points), row('radio pleas
news'::tsvector, '[(-90,-180),(90, 180)]'));
count
---
10
(1 row)
Using the sequential scan, still, everything seems fine.
However, if I now do the index scan (my function will be called 11 times,
returning false at the end), I get the following result:
test=# select count(*) from documents where to_document(words, points)
row('pleas radio news'::tsvector, '[(-90,-180),(90,180)]');
count
---
7
(1 row)
This seems strange, because the same query returned 10 rows (when I didn't
use the aggregate). If I issue queries that count the id column, I receive
the following:
test=# select count(id) from documents where
hybrid_index_query(to_document(words, points), row('radio pleas
news'::tsvector, '[(-90,-180),(90, 180)]'));
count
---
10
(1 row)
test=# select count(id) from documents where to_document(words, points)
row('pleas radio news'::tsvector, '[(-90,-180),(90,180)]');
count
---
10
(1 row)
These two queries do again return the same results.
Thus, I don't know, what's wrong here, does anybody know about that
behaviour, or is it my fault that the results are wrong, somehow?
Thanks in advance
I am guessing this is a bug in your code - have you used EXPLAIN to
verify that the second-to-last of the above queries is really hitting
your code? If so, I'd recommend attaching with gdb and setting a
breakpoint wherever you return the tuples, and then poke around...
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company
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Re: [HACKERS] Custom index structure and strange count problem
Hi,
thanks so far.
However, if I attach a Debugger (which I did in advance, too) and I use
explain, I get the same results.
My first guess in each case is always that it is my fault. However, I don't
know exactly, why this strange behaviour occurs here. The problem I have is
that EXPLAIN, too, always tells me that it uses an index scan (in the cases
where the query is supposed to use one).
The query plan looks exactly the same in any case (if I apply count(id) or
count(*), respectively). However, the results differ.
The query plan is also the same, if I use the select * or select id query
without applying an aggregate with the small difference that the aggregate is
used, where it is supposed to be.
I just thought, that somebody has already had problems with something like that
(actually it is no real problem, except that the aggregate applied to *
queries causes a different count).
The query data my index structure is called with, stays the same in all cases
(mentioned in the previous mail).
Does anybody have some hints according to which checks to perform in order to
determine the problem here?
Best regards
Carsten Kropf
Am 09.06.2010 um 19:09 schrieb Robert Haas:
On Wed, Jun 9, 2010 at 4:35 AM, Carsten Kropf ckro...@fh-hof.de wrote:
Hi *,
during the last few months I've been building a new index structure as part
of a research project.
Everything seems to work properly, however I have some strange issues with
the count sql command.
I introduced some custom structures (mainly document and hybrid_query) with
which my index access method is supposed to work.
There is an operator which is supposed to use my index structure (what
also works properly).
The function that maps to the operator is called hybrid_index_query,
which I use to compare my results given from the index with the real results
that are supposed to appear in the final result set.
Having described the outer circumstances (in a very short way), I will now
show the strange stuff that happens:
test=# select id from documents where hybrid_index_query(to_document(words,
points), row('radio pleas news'::tsvector, '[(-90,-180),(90, 180)]')) order
by id;
id
--
2137
2151
2168
2207
2208
2209
2210
2211
2266
2296
(10 rows)
This query takes a sequential scan and works properly (returning 10 rows).
test=# select id from documents where to_document(words, points)
row('pleas radio news'::tsvector, '[(-90,-180),(90,180)]') order by id;
id
--
2137
2151
2168
2207
2208
2209
2210
2211
2266
2296
(10 rows)
This query uses my index structure and returns the same result as in the
sequential scan above.
Until here, everything seems to work fine. However, if I issue the same
queries using the count aggregate function in SQL, there are some odd
results:
test=# select count(*) from documents where
hybrid_index_query(to_document(words, points), row('radio pleas
news'::tsvector, '[(-90,-180),(90, 180)]'));
count
---
10
(1 row)
Using the sequential scan, still, everything seems fine.
However, if I now do the index scan (my function will be called 11 times,
returning false at the end), I get the following result:
test=# select count(*) from documents where to_document(words, points)
row('pleas radio news'::tsvector, '[(-90,-180),(90,180)]');
count
---
7
(1 row)
This seems strange, because the same query returned 10 rows (when I didn't
use the aggregate). If I issue queries that count the id column, I receive
the following:
test=# select count(id) from documents where
hybrid_index_query(to_document(words, points), row('radio pleas
news'::tsvector, '[(-90,-180),(90, 180)]'));
count
---
10
(1 row)
test=# select count(id) from documents where to_document(words, points)
row('pleas radio news'::tsvector, '[(-90,-180),(90,180)]');
count
---
10
(1 row)
These two queries do again return the same results.
Thus, I don't know, what's wrong here, does anybody know about that
behaviour, or is it my fault that the results are wrong, somehow?
Thanks in advance
I am guessing this is a bug in your code - have you used EXPLAIN to
verify that the second-to-last of the above queries is really hitting
your code? If so, I'd recommend attaching with gdb and setting a
breakpoint wherever you return the tuples, and then poke around...
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company
--
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