This was explained by Tom Lane on the mailing list: what is “Recheck condition” and why is it needed? If the bitmap gets too large we convert it to “lossy” style, in which we only remember which pages contain matching tuples instead of remembering each tuple individually. When that happens, the table-visiting phase has to … Read more
I’m generalizing here, but… A clustered index seek is, for the most part, the best-case scenario. The only ways I can think of to improve performance would be: Update the query to return fewer rows/columns, if possible; Defragment or rebuild the index; Partition the index across multiple disks/servers. If it’s only returning 138 rows, and … Read more
Well, there seems to be nothing out there except a toolbar so I wrote my own mixin to give me an explain() method on my querysets: from django.db import connections from django.db.models.query import QuerySet class QuerySetExplainMixin: def explain(self): cursor = connections[self.db].cursor() cursor.execute(‘explain %s’ % str(self.query)) return cursor.fetchall() QuerySet.__bases__ += (QuerySetExplainMixin,) Hopefully this is useful to … Read more
I would appreciate any explanations to “Clustered Index Scan (Clustered)” I will try to put in the easiest manner, for better understanding you need to understand both index seek and scan. SO lets build the table use tempdb GO create table scanseek (id int , name varchar(50) default (‘some random names’) ) create clustered index … Read more
From the MySQL documentation: The query contained only aggregate functions (MIN(), MAX()) that were all resolved using an index, or COUNT(*) for MyISAM, and no GROUP BY clause. The optimizer determined that only one row should be returned. Basically this means your query uses data that is directly available to MySQL and the query will … Read more
The following are a few rules of thumb: Nested loop joins are preferred if one of the sides of the join has few rows. Nested loop joins are also used as the only option if the join condition does not use the equality operator. Hash Joins are preferred if the join condition uses an equality … Read more
The profiler trace puts it into perspective. Query A: 1.3 secs CPU, 1.4 secs duration Query B: 2.3 secs CPU, 1.2 secs duration Query B is using parallelism: CPU > duration eg the query uses 2 CPUs, average 1.15 secs each Query A is probably not: CPU < duration This explains cost relative to batch: … Read more
EXPLAIN won’t give you any indication of how long a query will take. At best you could use it to guess which of two queries might be faster, but unless one of them is obviously badly written then even that is going to be very hard. You should also be aware that if you’re using … Read more
A materialize node means the output of whatever is below it in the tree (which can be a scan, or a full set of joins or something like that) is materalized into memory before the upper node is executed. This is usually done when the outer node needs a source that it can re-scan for … Read more
EXPLAIN PLAN FOR In SQL Developer, you don’t have to use EXPLAIN PLAN FOR statement. Press F10 or click the Explain Plan icon. It will be then displayed in the Explain Plan window. If you are using SQL*Plus then use DBMS_XPLAN. For example, SQL> EXPLAIN PLAN FOR 2 SELECT * FROM DUAL; Explained. SQL> SELECT … Read more