On 7/16/24 17:43, Pierrick Chovelon wrote: > ... > > Quite fast as well... > > Have you got an idea on the initial issue ? Why when using a prepared > statement and a gin index the execution time "explode" ? > Something to do with the planner ? optimizer ? > > (We executed the same test with a btree index and execution times are > the same in both cases). >
The reason why the two queries end up with different plans is pretty simple - the condition ends up matching different operators, because of data type difference. In case of the prepared query, the (x <= 950000) matches <=(bigint,bitint) operator, and thus it matches the index. But that happens because the query is prepared with bigint parameter. For the regular query, the 950000 literal gets treated as int, the condition matches to <=(bigint,int) and that does not match the index - hence it's treated as a filter, not an index condition. If you cast the literal to bigint (by doing ::bigint) in the regular query, we end it'll use the same same plan as the prepared query - but that's the slower one, unfortunately :-( Which gets us to why that plan is slower, compared to the plan using fewer conditions. I think the problem is that <= 950000 matches most of the table, which means the GIN index will have to load and process a pretty long TID list, which is clearly not cheap. I don't think there's much you can do do - we don't consider this when matching conditions to the index, we simply match as many conditions as possible. And the GIN code is not smart enough to make judgements about which columns to process first - it just goes column by column and builds the bitmap, and building a bitmap on 95% of the table is costly. If this is a systemic problem for most/all queries (i.e. it's enough to have a condition on line_id), I believe the +0 trick is a good way to make sure the condition is treated as a filter. regards -- Tomas Vondra EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
