2017-09-21 12:52 GMT+02:00 Subramaniam C <subramaniam31...@gmail.com>:
> Hi > > I wanted to query top 20 rows by joining two tables, one table having > around 1 lac rows and other table having 5 lac rows. Since I am using ORDER > BY in the query so I created compound index with the columns being used in > ORDER BY. Initially index size was 939 MB. > > Then I ran EXPLAIN(ANALYZE,BUFFERS) for this query which took around 20 > secs as it was not using the compound index for this query. So I drop this > index and created again. The index size now got reduced to 559 MB. > > After this if I ran the EXPLAIN(ANALYZE,BUFFERS) for this query it was > using the index and took only 5 secs. > > Can you please explain how the index size got reduced after recreating it > and how the query started using the index after recreating? > > The index can be bloated - when you recreate it or when you use REINDEX command, then you remove a bloat content. VACUUM FULL recreate indexes too. Fresh index needs less space on disc (the read is faster), in memory too and has better structure - a access should be faster. > Thanks and Regards > Subramaniam >
