Is it a bad symptom that non-clustered index scan cost is 53% ?
That depends on your query. The total query always costs 100%. So if you have a query like
SELECT Name from Customers WHERE ID = 3
than the index scan or seek may even cost 100%. That doesn't mean it's a bad thing. If you want a clear answer about you're query then you should at least post the query itself.
SQL Server will not use non-clustered index with key/bookmark lookup in the case if it expects iterator to return more than a few % of the total rows from the table.
Related
I am using Postgres database , I am trying to see the difference between Index Scan and Sequential scan on table of 1000000 rows
Describe table
\d grades
Then explain analyze for rows between 10 and 500000
explain analyze select name from grades where pid between 10 and 500000 ;
Then explain analyze for rows between 10 and 600000
explain analyze select name from grades where pid between 10 and 600000 ;
The strange for me why it made Index scan on first query and
sequential scan in the second although they query by the same column
which it contained in the index .
If you need only a single table row, an index scan is much faster than a sequential scan. If you need the whole table, a sequential scan is faster than an index scan.
Somewhere between that is the turning point where PostgreSQL switches between these two access methods.
You can tune random_page_cost to influence the point where a sequential scan is chosen. If you have SSD storage, you should set the parameter to 1.0 or 1.1 to tell PostgreSQL that index scans are cheaper on your hardware.
PostgreSQL uses a cost based optimizer, not a rule based optimizer. If you take the estimated cost of the index scan, 18693, and scale it up linearly by the ratio of the expected rows between the two plans (which is not exactly what the planner does, but should be a good enough first approximation) you get 22330. That is higher than the expected cost of the seq scan, 21372, so it chooses the seq scan.
If you scale the index-scan actual time up the same way, you get 89ms, which is slightly faster than the seq scan actually was. So maybe the planner made a very slight error here, but it is certainly nothing to worry about in practice.
If the difference in run times were a factor of 10, rather than 10%, that might be worth investigating further.
its because If the SELECT returns more than approximately 5-10% of all rows in the table, a sequential scan is much faster than an index scan. and your second query hit that threshold; because you are fetching more rows
There is scenario, I have table with 40 columns and I have to select all data of a table (including all columns). I have created a clustered index on the table and its including Clustered Index Scan while fetching full data set from the table.
I know that without any filter or join key, SQL Server will choose Clustered Index Scan instead of Clustered Index Seek. But, I want to have optimize execution plan by optimizing Clustered Index Scan into Clustered Index Seek. Is there any solution to achieve this? Please share.
Below is the screenshot of the execution plan:
Something is not quite right in the question / request, because what you are asking for will perform badly. I suspect it comes from mis-understanding what a clustered index is.
The clustered index - which is perhaps better stated as a clustered table - is the table of data, its not separate to the table, it is the table. If the order of the data on the table is already based on ITEM ID then the scan is the most efficient access method for your query (especially given the select *) - you do not want to seek in this scenario at all - and I don't believe that it is your scenario due to the sort operator.
If the clustered table is ordered based on another field, then you would need an additional non-clustered index to provide the correct order. You would then try to force a plan which was a non-clustered index scan, nested loop to a clustered index seek. That can be achieved using query hints, most likely an INNER LOOP JOIN would cause the seek - but a FORCESEEK also exists which can be used.
Performance wise this second option is never going to win - you are in effect looking at a tipping point notion (https://www.sqlskills.com/blogs/kimberly/the-tipping-point-query-answers/)
Well, I was trying to achieve the same, I wanted an index seek instead of an index scan on my top query.
SELECT TOP 5 id FROM mytable
Here is the execution plan being shown for the query:
I even tried the Offset Fetch Next approach, the plan was same.
To avoid a index scan, I included a fake primary key filter like below:
SELECT TOP 5 id FROM mytable where id != 0
I know, I won't have a 0 value in my primary key, so I added it in top query, which was resolved to an index seek instead of index scan:
Even though, the query plan comparison gives operation cost as similar to other, for index seek and scan in this regard. But I think to achieve index seek this way, it is an extra operation for the db to perform because it has to compare whether the id is 0 or not. Which we entirely do not need it to do if we want the top few records.
I have a table which has non clustered index on PFX,EFF_DT,TERM_DT. The execution plan shows RID LookUp heap cost is 99%, instead of index scan. I want to know the reason why not index scan is not in execution plan, and is RID LookUp is good approach.
SELECT DISTINCT
ID
,PFX
,EFF_DT
,ID1
,TERM_DT
,RULE
,EXP_CAT
,ACCT_CAT
,OPTS
,RULE_ALT
,RULE_ALT_COND
FROM TempMaster
WHERE PFX = 'I004'
ORDER BY EFF_DT DESC
I want to know the reason why not index scan is not in execution plan
SQLSERVER is a cost based optimizer and it tries to choose a good plan in reasonable amount of time..
is RID LookUp is good approach
RID lookup is not always a good approach,since RID lookups are random seeks and they affect IO activity..
I would not worry if this query executes once a day..If this query is more frequent,i would avoid rid looup by including those columns in nonclustered index as well
I am under the impression that column order for index matters. So an index on columns (A,B) would not be used for SELECTs WHERE B=yy. (not that it matters I think, but assume the index is non-clustered)
But I just ran a query that fits this form on a table with an index just like above and got unexpected results. According to sql server management studio, the actual query plan used involved using the non-clustered index.
Why could this have happened?
It probably reported an Index Scan which is comparable to a full table scan. Imagine an address book indexed by (as most are) LastName, FirstName. A query for "Doe,John" will result in a Index Seek, while a query for "John" would result in a Index Scan.
We have a sql query as follows
select * from Table where date < '20091010'
However when we look at the query plan, we see
The type of query is SELECT.
FROM TABLE
Worktable1.
Nested iteration.
Table Scan.
Forward scan.
Positioning at start of table.
Using I/O Size 32 Kbytes for data pages.
With MRU Buffer Replacement Strategy for data pages.
which seems to suggest that a full table scan is done. Why is the index not used?
If the majority of your dates are found by applying < '20091010' then the index may well be overlooked in favour of a table scan. What is your distribution of dates within that table? What is the cardinality? Is the index used if you only select date rather than select *?
Unless the index is covering *, the optimizer realizes that a table scan is probably more efficient than an index seek/scan and bookmark lookup. What's the expected selectivity of the date range? Do you have a primary key defined?