I have a table in PostgreSQL 9.2 that has a text
column. Let's call this text_col
. The values in this column are fairly unique (may contain 5-6 duplicates at the most). The table has ~5 million rows. About half these rows contain a null
value for text_col
. When I execute the following query I expect 1-5 rows. In most cases (>80%) I only expect 1 row.
explain analyze SELECT col1,col2.. colN
FROM table
WHERE text_col = 'my_value';
A btree
index exists on text_col
. This index is never used by the query planner and I am not sure why. This is the output of the query.
Seq Scan on two (cost=0.000..459573.080 rows=93 width=339) (actual time=1392.864..3196.283 rows=2 loops=1)
Filter: (victor = 'foxtrot'::text)
Rows Removed by Filter: 4077384
I added another partial index to try to filter out those values that were not null, but that did not help (with or without text_pattern_ops
. I do not need text_pattern_ops
considering no LIKE
conditions are expressed in my queries, but they also match equality).
CREATE INDEX name_idx
ON table
USING btree
(text_col COLLATE pg_catalog."default" text_pattern_ops)
WHERE text_col IS NOT NULL;
Disabling sequence scans using set enable_seqscan = off;
makes the planner still pick the seqscan
over an index_scan
. In summary...
A partial index is a good idea to exclude half the rows of the table which you obviously do not need. Simpler:
CREATE INDEX name_idx ON table (text_col)
WHERE text_col IS NOT NULL;
Run ANALYZE table
after creating the index. (autovacuum
does that automatically after some time if you don't do it manually, but if you test right after creation, your test will fail.)
Then, to convince the query planner that a particular partial index can be used, repeat the WHERE
condition in the query - even if it seems redundant:
SELECT col1,col2, .. colN
FROM table
WHERE text_col = 'my_value'
AND text_col IS NOT NULL; -- repeat condition
However, keep in mind that the predicate must match the conditions used in the queries that are supposed to benefit from the index. To be precise, a partial index can be used in a query only if the system can recognize that the
WHERE
condition of the query mathematically implies the predicate of the index. PostgreSQL does not have a sophisticated theorem prover that can recognize mathematically equivalent expressions that are written in different forms. (Not only is such a general theorem prover extremely difficult to create, it would probably be too slow to be of any real use.) The system can recognize simple inequality implications, for example "x < 1" implies "x < 2"; otherwise the predicate condition must exactly match part of the query'sWHERE
condition or the index will not be recognized as usable. Matching takes place at query planning time, not at run time. As a result, parameterized query clauses do not work with a partial index.
As for parameterized queries: again, add the (redundant) predicate of the partial index as an additional, constant WHERE
condition, and it works just fine.
Update: Postgres 9.6 or later largely improves chances for index-only scans. See: