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Top Forums Programming DB2 - Performance Issue using MERGE option Post 303018332 by rbatte1 on Monday 4th of June 2018 07:03:36 AM
Old 06-04-2018
I expect that this is doing what you asked, and although it does seem a sensible query, it is not being very practical. I think that your request is going to build some huge temporary tables to work out the various clauses before applying any inserts/updates. You may even fill your temporary table tablespace before your MERGE completes this step.


You might have some success with indices though. What indices do you have on table schemaname.Customer_Staging and schemaname.Customer_Staging?
  • If you don't have a single index for the three columns you mention in the SELECT DISTINCT, then you might do a full table scan of schemaname.Customer_Staging.
  • If you don't have a single index for all the columns in the whole query (e.g A.Name, B.Name, A.Load_dt etc. ) then you might do a full table scan of the appropriate table.
Any reason to do a full table scan that these scales will be bad. An index might take a while to build and needs to have space, but then your MERGE should run better. I've had something that took me 20 minutes to build an index then the process I wanted ran in about an hour, after which I dropped the index again.because it was a one-off report. The run without the index was abandoned after over 22 hours (someone set if off and went home)

Is there some sort of query profiler you can use to consider this? MSSQLServer has one, Oracle has one so I'm sure that DB2 will have one, but I've not ever used it. You need to avoid a full table scan when dealing with this volume of data.



I hope that this helps,
Robin

Last edited by rbatte1; 06-04-2018 at 08:03 AM.. Reason: Spelling correction.
 

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CREATE TABLE 
AS(7) SQL Commands CREATE TABLE AS(7) NAME
CREATE TABLE AS - create a new table from the results of a query SYNOPSIS
CREATE [ [ LOCAL ] { TEMPORARY | TEMP } ] TABLE table_name [ (column_name [, ...] ) ] AS query DESCRIPTION
CREATE TABLE AS creates a table and fills it with data computed by a SELECT command. The table columns have the names and data types asso- ciated with the output columns of the SELECT (except that you can override the column names by giving an explicit list of new column names). CREATE TABLE AS bears some resemblance to creating a view, but it is really quite different: it creates a new table and evaluates the query just once to fill the new table initially. The new table will not track subsequent changes to the source tables of the query. In contrast, a view re-evaluates its defining SELECT statement whenever it is queried. PARAMETERS
[LOCAL] TEMPORARY or [LOCAL] TEMP If specified, the table is created as a temporary table. Refer to CREATE TABLE [create_table(7)] for details. table_name The name (optionally schema-qualified) of the table to be created. column_name The name of a column in the new table. Multiple column names can be specified using a comma-delimited list of column names. If col- umn names are not provided, they are taken from the output column names of the query. query A query statement (that is, a SELECT command). Refer to SELECT [select(7)] for a description of the allowed syntax. DIAGNOSTICS
Refer to CREATE TABLE [create_table(7)] and SELECT [select(7)] for a summary of possible output messages. NOTES
This command is functionally equivalent to SELECT INTO [select_into(7)], but it is preferred since it is less likely to be confused with other uses of the SELECT ... INTO syntax. COMPATIBILITY
This command is modeled after an Oracle feature. There is no command with equivalent functionality in SQL92 or SQL99. However, a combina- tion of CREATE TABLE and INSERT ... SELECT can accomplish the same thing with little more effort. HISTORY
The CREATE TABLE AS command has been available since PostgreSQL 6.3. SEE ALSO
CREATE TABLE [create_table(7)], CREATE VIEW [create_view(l)], SELECT [select(l)], SELECT INTO [select_into(l)] SQL - Language Statements 2002-11-22 CREATE TABLE AS(7)
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