Hi,
I have a single table with about 10 million rows, and two indexes. Index A is on a column A with 95% null values. Index B is on a column B with about 10 values, ie. About a million rows of each value.
When I do a simple query on the table (no joins) with the following condition:
A is null AND
B = ‘21’
it uses the correct index, index B. However, when I add a limit clause of 15, postgres decides to do a sequential scan :s. Looking at the results from explain:
"Limit (cost=0.00..3.69 rows=15 width=128)"
" -> Seq Scan on my_table this_ (cost=0.00..252424.24 rows=1025157 width=128)"
" Filter: ((A IS NULL) AND ((B)::text = '21'::text))"
It appears that postgres is (very incorrectly) assuming that it will only have to retrieve 15 rows on a sequential scan, and gives a total cost of 3.69. In reality, it has to scan many rows forward until it finds the correct value, yielding very poor performance for my table.
If I disable sequential scan (set enable_seqscan=false) it then incorrectly uses the index A that has 95% null values: it seems to incorrectly apply the same logic again that it will only have to retrieve 15 rows with the limit clause, and thinks that the index scan using A is faster than index scan B.
Only by deleting the index on A and disabling sequential scan will it use the correct index, which is of course by far the fastest.
Is there an assumption in the planner that a limit of 15 will mean that postgres will only have to read 15 rows? If so is this a bad assumption? If a particular query is faster without a limit, then surely it will also be faster with the limit.
Any workarounds for this?
Thanks
David
