Bug? relpages, reltuples resets to zero

Hi,

there seems to be a problem with the relation statistics in
pg_class. Could someone explain why this happens?

doc=> vacuum;
VACUUM
doc=> select relname, relpages, reltuples from pg_class
doc-> where relname = 'doc_wordref';
relname |relpages|reltuples
-----------+--------+---------
doc_wordref| 1099| 136027
(1 row)

-- ******** That's right

doc=> explain select distinct refpage from doc_wordref
doc-> where refword ~ '^a';
NOTICE: QUERY PLAN:

Unique
-> Sort
-> Index Scan using doc_wordref_word_idx on doc_wordref

EXPLAIN

-- ******** As expected

doc=> select distinct refpage from doc_wordref
doc-> where refword ~ '^a';
refpage
-------
2
3
...
(164 rows)

doc=> select relname, relpages, reltuples from pg_class
doc-> where relname = 'doc_wordref';
relname |relpages|reltuples
-----------+--------+---------
doc_wordref| 0| 0
(1 row)

-- ******** Ooops - where are they gone?

doc=> explain select distinct refpage from doc_wordref
dos-> where refword ~ '^a';
NOTICE: QUERY PLAN:

Unique
-> Sort
-> Index Scan using doc_wordref_word_idx on doc_wordref

-- ******** Doesn't matter in the same connection, so reconnect

EXPLAIN
doc=> \c -
connecting to new database: doc
doc=> explain select distinct refpage from doc_wordref
dos-> where refword ~ '^a';
NOTICE: QUERY PLAN:

Unique
-> Sort
-> Seq Scan on doc_wordref

-- ******** Boom

EXPLAIN
doc=>

Why does the SELECT throw away the information about the
number of pages and tuples from pg_class? Is this a bug or a
feature? If it's a feature, how can I disable it? It is
reproduceable.

Jan

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