Hi, all
When create or refresh a Materialized View, if the view’s query has order by, we may sort and insert the sorted data into view. Create Materialized View mv1 as select c1, c2 from t1 order by c2; Refresh Materialized View mv1; And it appears that we could get ordered data when select from Materialized View; Select * from mv1; But it’s not true if we use other access methods, or we choose a parallel seqscan plan. A non-parallel seqscan on heap table appears ordered as we always create new rel file and swap them, in my opinion, it’s more like a free lunch. So, conclusion1: We couldn’t rely on the `ordered-data` even the mv’s sql has order by clause, is it right? And if it’s true, shall we skip the order by clause for Materialized View when executing create/refresh statement? Materialized View’s order by clause could be skipped if 1. Order by clause is on the top query level 2. There is no real limit clause The benefit is the query may be speeded up without sort nodes each time creating/refreshing Materialized View. A simple results: create table t1 as select i as c1 , i/2 as c2 , i/5 as c3 from generate_series(1, 100000) i; create materialized view mvt1_order as select c1, c2, c3 from t1 order by c2, c3, c1 asc Without this patch: zml=# refresh materialized view mvt1_order; REFRESH MATERIALIZED VIEW Time: 228.548 ms zml=# refresh materialized view mvt1_order; REFRESH MATERIALIZED VIEW Time: 230.374 ms zml=# refresh materialized view mvt1_order; REFRESH MATERIALIZED VIEW Time: 217.079 ms With this patch: zml=# refresh materialized view mvt1_order; REFRESH MATERIALIZED VIEW Time: 192.409 ms zml=# refresh materialized view mvt1_order; REFRESH MATERIALIZED VIEW Time: 204.398 ms zml=# refresh materialized view mvt1_order; REFRESH MATERIALIZED VIEW Time: 197.510 ms Zhang Mingli www.hashdata.xyz
v1-01-Skip-Orderby-clause-execution-for-Materialized-Views.patch
Description: Binary data