Thread: [PERFORM] Query regarding EXPLAIN (ANALYZE,BUFFERS)
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?
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?
Thanks and Regards
Subramaniam
2017-09-21 12:52 GMT+02:00 Subramaniam C <subramaniam31784@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 RegardsSubramaniam