On fös, 2006-02-17 at 12:06 -0500, Emi Lu wrote: > >>>>In another way, whenever we "delete/truncate and then insert" data into > >>>>a table, it is better to "vacuum anaylze"? > >>>> > ... > >> > >>So, your suggestion is that after the population of table A, the query > >>planner should be able to find the most efficient query plan because we > >>do truncate but not delete, and we do not need to do vacuum analyze at > >>all, right? > >> > >> > ... > > Thank you gnari for your answer. But I am a bit confused about not running vacuum but only "analyze". Can I seperate these two operations? I guess "vacuum analyze" do both vacuum and analyze. > Or "EXPLAIN ANALYZE" can do it for me? VACUUM ensures that dead rows can be reused. Dead rows are created by DELETE and UPDATE. If you have done a significant number of DELETEs or UPDATEs, you might want to VACUUM ANALYZE collect statistical information about your tables. this helps the planner make good plans. After having changed your data significantly, you might want to ANALYZE, for example after lots of INSERTs, UPDATEs or DELETEs TRUNCATE does not create dead rows, so you do not need to VACUUM just because of that, but you still might have to ANALYZE. If you TRUNCATE a table and then repopulate it with similar data as before, you do not have to ANALYZE, as plans based on the old statistics would assumedly be just as good. EXPLAIN dislays the plan that will be chosen for a query, along with some estimated cost information. EXPLAIN ANALYZE actually executes the query, and shows same info as EXPLAIN, and in addition actual cost information Hope this makes it more clear gnari
