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

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2
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1answer
435 views

Is there any way to save the execution plan in a file in PostgreSQL?

By saving a plan in a file, I don't mean to view it, but force the optimizer to use the saved plan to execute some query. For example, if the optimizer chooses to do a hash join for a two-table ...
3
votes
2answers
497 views

Multicolumn statistics on large table yielding poor estimations

I have a rather large table (~200 million rows), and although the statistics are up to date using WITH FULLSCAN, is it possible that my histogram (limited to 200 steps) is simply too broad for the ...
2
votes
0answers
210 views

Does Oracle ADO.NET varchar parameter size influence query plan reuse?

I am considering to apply a fix which will generate constant parameter sizes for (n)varchar parameters. The application is based on NHibernate with the old OracleClientDriver (using Microsoft Oracle ...
2
votes
0answers
146 views

Multi-Statement Template Plan Guide

A piece of, um, software seems to be issuing ~20k queries of this form every few seconds: DELETE FROM macVendor WHERE companyId=N' 08-00-'; INSERT INTO macVendor (companyId,organization) VALUES (N' ...
6
votes
1answer
364 views

Capture Complex Actual Execution Plans

I have a stored procedure that among other things builds and executes a large number of Dynamic SQL statements. (Don't hate me, I didn't write it.) I would like to retrieve the actual execution plan ...
4
votes
1answer
1k views

Non-Clustered Index key columns to cover a variable where clause

To whoever reads this, sorry for the long question and thanks so much for your time. I'm needing some help on understanding what columns should be key/non key in the following situation, and what ...
2
votes
0answers
49 views

Identical query & instance but other execution plans & performance [duplicate]

I have 2 identical queries . One is executed with parameters, the other is executed with hardcoded values. Somehow sql server is creating 2 different execution plans for the 2 queries. The first one ...
4
votes
1answer
340 views

Why clustered index scan?

I just started to learn to optimize my queries and to analyze their query plans. I thought this query would generate nonclustered index seek + key lookup. SELECT ct.* FROM Person.ContactType AS ct ...
3
votes
1answer
115 views

Understanding SQL Server plan cache

I've been reading about the SQL Server plan cache and optimizing for adhoc workloads, etc. I'm fairly new to the topic and have a few general questions to help paint a better picture: Suppose I have ...
1
vote
0answers
92 views

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 ...
3
votes
1answer
121 views

Simple SELECT takes forever, but works as a subquery. Why?

I'm running a MySQL 5.1 server and I have a MyISAM table called my_table with nearly 5 million records. Its structure is the following: ...
8
votes
1answer
406 views

Does SQL Sentry Plan Explorer count the reads in UDFs?

I have a query like this: select dbo.fn_complexFunction(t.id) from mytable t In SQL Sentry Plan Explorer, I noticed I have to run Get Estimated Plan in SQL Sentry to make the Query Plan include the ...
2
votes
1answer
2k views

How bad is query plan pollution (too many equivalent plans in cache)?

Question currently affects C#/.NET, DB access based on ADO.NET and SQL Server 2008 R2, but I think it applies to other databases as well. I noticed some old modules of a system have non-optimal SQL ...
1
vote
1answer
166 views

Estimating a Size of Joining a Relation with itself

I'm studying size estimation of logical query plans in order to select a physical query plan. I was wondering what is the size of joining (natural join) a relation to itself? e.g R(a,b) JOIN R(a,b)
1
vote
1answer
6k views

Different execution plan for the same query if I change a value in the predicate

I am getting two different execution plans for the same query only by changing the value in the predicate, the first one uses nested loops and the other one hash join which takes a lot longer. ...
0
votes
1answer
314 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 ...
5
votes
3answers
4k views

SQL Server 2005 get execution plan of a overnight stored procedure

There is a overnight job runs long time. I have used profiler to get the trace and identified a statement (in a stored procedure) that takes most of execution time. Now I need to see the execution ...
1
vote
1answer
102 views

A query executed in a backup of a database is performing better than if executed in the original database

We have two databases with the same data, let's call it Database A which is the original and Database B which is the backup from Database A. A query is performing much better in Database B for example ...
2
votes
0answers
233 views

Postgres - Partitioning old tables, partition query planning, optimisation

I've been trying to optimise our database, but not being a DBA and not having done a whole lot of tuning before I'm struggling a little with "too many options". I've already run pg_tune and bumped a ...
1
vote
2answers
2k views

Improve performance with the WHERE NOT IN sub-select clause

In the following query, I have to count transactions for each customer. [EDIT]However, I have to exclude from the result set entirely, customers that have a transaction older than one year. ...
3
votes
1answer
237 views

Optimizing complex join predicates [closed]

Table Definitions CREATE TABLE [dbo].[Pin_Mtg] ( [MtgId] bigint NULL, [CntyCd] char(5) NOT NULL, [BatchDt] int NOT NULL, [BatchSeq] int NOT NULL, [MtgSeq] tinyint NOT NULL, ...
2
votes
2answers
980 views

How can you tell when a query plan reuse is causing performance problems

When investigating a performance issue, a workmate suggested running DBCC FREEPROCCACHE to clear the plan cache. He came to this conclusion after noticing performance improved after a reboot. This ...
16
votes
4answers
927 views

Execution plan vs STATISTICS IO order

SQL Server graphical execution plans read right to left and top to bottom. Is there a meaningful order to the output generated by SET STATISTICS IO ON? The following query: SET STATISTICS IO ON; ...
3
votes
2answers
3k views

SQL Query too slow on SUM function

I've a table with around 32 million rows having clustered unique index on CountryID,RetailerID,ProductID,DateID,EventID,TypeID and query is SELECT f.RetailerID , TypeID ...
11
votes
2answers
7k views

How to optimise T-SQL query using Execution Plan

I have a SQL query that I have spent the past two days trying to optimise using trial-and-error and the execution plan, but to no avail. Please forgive me for doing this but I will post the entire ...
9
votes
2answers
3k views

When does sp_executesql refresh the query plan?

You'll have to forgive my naivety as I'm not a DBA but my understanding is that over time the statistics of a database change and a stored procedure must be recompiled to keep the query plan up to ...
1
vote
2answers
3k views

Updating a table efficiently using JOIN

I have a table that has the details of households and another that has the details of all the persons associated with the households. For the household table I have a primary key defined using two ...
5
votes
2answers
3k views

Should I refresh query plan cache

Please let me explain my problem and situation: I have a web application - MVC3, MSSQL Server 2005, LinqToSQL. It has been running great until one fine morning I pushed a lot of rows to a table that ...
6
votes
0answers
178 views

Need help understanding query plan: estimates vs. actuals [closed]

I'd like to understand a section of the query plan that seems to mess up my query. The picture shows that there's a clustered index seek, followed by compute scalar, followed by sort. The part ...
6
votes
2answers
743 views

Query Plans not retained insufficient memory errors

We have been experiencing memory issues with SQL Server. We first realised we had a problem when we started getting timeouts and login errors: A connection was successfully established with the ...
2
votes
1answer
564 views

Oracle 11g bug ? not returning the record until I triggered index to invisible and then to visible

We are using Oracle 11g, 11.2.0.3. We know a record exists in a table but a select is not returning it for some odd reason. The execution plans the general query, not forcing any index, shows that ...
2
votes
1answer
271 views

Relative cost of two similar queries involving XML columns

I have two queries as shown below which do the same thing. Here xmlcolumn is a column with datatype XML. I use these queries to search for a string anywhere in the XML column. I checked the execution ...
2
votes
2answers
82 views

On which server does a query execute?

Given two Microsoft SQL servers: Say ServerA has a view titled "View1": SELECT * FROM ServerB.database.dbo.Table1 WHERE Column1 IN (1,2,3) and a stored procedure "sp_Foo": SELECT * FROM View1 WHERE ...
6
votes
1answer
432 views

Why does the plan cache contain estimated and not actual execution plans?

I can think of many reasons behind this decision of storing estimated plans in the plan cache and not the actual plan. But I can't find the "correct" answer.
2
votes
1answer
117 views

Execution Plan not matching Stored Proc

Below is a stored proc to count the number of widgets made in a month. If no widgets are made, no records will exists. The execution plan shows an INNER JOIN of the M and A tables, where in the ...
35
votes
4answers
27k views

Index Seek vs Index Scan

Looking at an execution plan of a slow running query and I noticed that some of the nodes are index seek and some of them are index scan. What is the difference between and index seek and an index ...
7
votes
3answers
1k views

Are SQL Server statistics stored in database or buffer pool?

Just wondering are statistics kept in the database but not in the memory? If I backup/restore the database from a prod server to a development server, would it keep the same statistics so that ...
3
votes
1answer
546 views

How to determine cause of runtime increase given two query plans with SpillToTempDb warning

Two actual query plans were captured for two executions of one query: Plan "Fast" took ~1s and occurs about 90% of the time. Plan "Slow" took ~16s. Since the graphical plans look ...
5
votes
1answer
212 views

Are database query optimizers aware of storage performance differences?

As I understand it, the query optimizer in SQL Server (or any other RDBMS, really) isn't aware of the performance of the storage underneath the database, and will make decisions as if all storage has ...
7
votes
1answer
2k views

How does SQL Server generate a query execution plan that adds up to 6,000%? [duplicate]

Today I was on The Heap and was looking at a query plan for something I thought could be improved. However, it created something that shook my belief in the SQL Server query optimizer. Can I still ...
6
votes
4answers
3k views

How can I remove a bad execution plan from Azure SQL Database?

DBCC FREEPROCCACHE doesn't work in Azure SQL DB. How else can I force a plan to kick itself out of the cache in a way that won't hurt a production system (i.e. I can't just go alter tables willy ...
3
votes
1answer
543 views

sql server partitioned view execution plan questions

I have created a partitioned view for 3 tables tb_sales2010,tb_sales2011,tb_sales2012 using check constraint (respectivly for each table) datepart(year,[Date])=(2010) datepart(year,[Date])=(2011) ...
4
votes
1answer
1k views

SQL Server 2008 query planner failing after index drop & recreate

Recently we ran a script to our production DB that would dynamically drop and recreate hundreds of indexes as filtered indexes. While this script had run perfectly in all other previous tests, now ...
6
votes
3answers
9k views

PostgreSQL Sequential Scan instead of Index Scan Why?

Hi All I've got a problem with my PostgreSQL database query and wondering if anyone can help. In some scenarios my query seems to ignore the index that I've created which is used for joining the two ...
2
votes
1answer
2k views

Fast paginated result, for various filter clauses

I have been working on obtaining a paginated result from a table in my database (11g). While I have a query that does work (ie, the result is correct) it does not perform as well as I'd like to and I ...
13
votes
1answer
1k views

Should I be alarmed by this NO JOIN PREDICATE warning?

I'm troubleshooting the bits and pieces of a poorly-performing stored procedure. This section of the procedure is throwing a NO JOIN PREDICATE warning select method = case ...
30
votes
3answers
11k views

Parameter Sniffing vs VARIABLES vs Recompile vs OPTIMIZE FOR UNKNOWN

So we had a long running proc causing problems this morning (30 sec + run time). We decided to check to see if parameter sniffing was to blame. So, we rewrote the proc and set the incoming ...
4
votes
2answers
228 views

Performance of different precedence pseudocolumn solutions with LIMIT

I was wondering about the relative performance of the solutions provided in the answers to a question on stackoverflow, I decided to run some tests. The OP wanted to get the first matching row given ...
3
votes
1answer
615 views

Execution plan in simple query in PgSQL depends on offset clause, why?

Update on 2013-01-16 11:08: see below I am working on PostgreSQL 8.4.14 with the following table fed with 53k lines: CREATE TABLE botany.taxon_data_cache ( taxon_id int NOT NULL, ...
4
votes
2answers
220 views

Performance on SQL Join

I have a query which takes ages to run mainly due to a join between 2 tables on a PK and FK. The Key is prefixed with 2 letters (which is always the same, RN) followed by 7 digits So RN1234567 for ...