The strategy selected by the query optimiser to process a query.

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What to look for in query execution plans? [closed]

Assuming the query execution is slow, I have collected the xml plan. What should I look for in the actual execution plan. What are the top 10 things I need to watch out for? I know this is a broad ...
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why the optimizer doesn't choose the optimal plan [closed]

I know, if the statistic is not updatad, it is possible, that the optimizer doesn't choose the optimal execution plan? Does any other reason/reasons exist?
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100 views

How many LEFT JOINS make a VIEW unusable?

I have a view which involves LEFT JOINS on 20+ tables to a base table but when querying this view and looking at the execution plans I am getting something totally unexpected. For example, with the ...
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2answers
47 views

Identical SQL Server databases hosted in the same physical DB server

We have hosted similar SQL Server 2014 databases TestDB and TESTDB_Copy on the same physical server. The two databases are almost the same and there may be minor differences in the number of records ...
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268 views

Oracle 10g: same query, different plans on copies of the same DB

I have a query joining a number of long tables (primary key in one table, indexed column in another) which results in different plans on different copies of the same database. The structure of copies ...
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66 views

Access and filter in oracle plan

I came to a execution plan where oracle choose to access the index but instead of using the columns as predicate it only filter by them. My question is what are the reasons that can cause that(all ...
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238 views

Comparing execution plans [closed]

How does the optimizer compare different execution plans of the same query? Let's say we have two complex and structurally different execution plans, how can I know if they are providing the same ...
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1answer
254 views

SQL Server Index Scan Actual Executions

I have a fairly complicated query and running it on two databases with same schemas, but with different data produces different result. On database D1 the elapsed time is around 1 second, and the ...
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654 views

Query performance and join hints, plan cost, and duration

Having some trouble identifying why a query's duration would decrease when using OPTION (HASH JOIN) or OPTION (MERGE JOIN), although plan cost increases. Background I have a reporting database using ...
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19 views

Merge join turns to hash match when adding a 2nd join

I would like to educate myself on what goes on under the hood in SQL Server, so I'm about to delve into Grant Fritchey's book Dissecting SQL Server Execution Plans. As it's 181 pages I just wanted to ...
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57 views

Eager Spool 100%

I have a query like below, when I execute it, the parallel query always takes very long time to finish, I found there is index spool(Eager Spool) which costs 100% in execution plan... How can I ...
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45 views

Why does this plan cost even though there are no rows?

I execute a Batch-Operation. The whole execution needs about 60s. The following statement needs 75.6% of the whole Batch-Operation: update WW_Pos set LieferantID=null where LieferantID in (select ...
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54 views

SQL query running slow compared to its earlier execution time

One of our application using an SQL query is reported running slow. Current execution time:- 15 minutes and so Earlier execution time:- 30 seconds. SQL-Sentry Plan for the slow query I've ...
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76 views

intermittent performance issue - query with multiple CTE and trashed execution plan

I'm trying to optimize a SQL query that shows variable performance (less than 2 to more than 15s). Overview This is a ColdFusion website, with a SQL Server 2008 R2 backend The query is part of the ...
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1answer
65 views

Optimizing a DMV query

I have found a query that is a major performance issue in my environment. I looked at the actual execution plan for this query and found out the main problem is the remote scan (98%). Its remote ...
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1answer
201 views

What are the OMem and 1Mem columns in the plan output?

The plan output, for example SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR( '' ,NULL,'ALLSTATS LAST')); has the columns OMem and 1Mem. What is their meaning? I could not find any reference or ...
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79 views

Index Tuning Question

I have the following query: SELECT T1.ITEMID, T1.ITEMTYPE, T1.PURCHMODEL, T1.HEIGHT, T1.WIDTH, T1.SALESMODEL, T1.COSTGROUPID, T1.REQGROUPID, T1.EPCMANAGER, ...
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1answer
88 views

same query different servers,different execution plan and time

I have this query and it runs less than 2 minutes on Test but on Prod it takes 10 minutes. Both are the same version,CPU,RAM,OS the same. I have four tempdb files on Prod but one one Test. I ...
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29 views

Analize Query in MySQL Server 5.1

I try use MySQL Workbench 6 to analyze the cost, but they don't show the cost of query. How i get this costs?