Error Message : The query processor ran out of internal resources and could not produce a query plan. This is a rare event and only expected for extremely complex queries or queries that reference a very large number of tables or partitions. Please simplify the query.

Error Message:
The query processor ran out of internal resources and could not produce a query plan. This is a rare event and only expected for extremely complex queries or queries that reference a very large number of tables or partitions. Please simplify the query. If you believe you have received this message in error, contact Customer Support Services for more information.

As specified in the error message, its a rare event or exception. The important message from the exception is that “Please simplify the query”. When the query is complex, the relational engine would not be able to create a query plan due to various reasons like the environment resource limitation, reduced capacity etc. and end up with the above exception.

The case I observed is with a dynamic query execution. I do not exactly going to give the query, however tries to provide a more like scenario. I wanted to create a dynamic query with an IN condition where the list of items are separated with a comma like below.

declare @sql varchar(max)
Declare @values varchar(max)='val1','val2','val3'......'valhugenumber'
Select @sql = 'Select * from ' + @tablename + ' where ' + @columnname + ' in ('+@values+')'
Exec(@sql)


When executing this in one of environments (lower), the error is produced, however, the other interesting thing is its not happening in few other environments(eg. Production). As mentioned above, that is because it depends on the resources involved in various environment matters in producing the query plan.

Resolution:

From the analysis, the issue is associated with long (really long) list of values and when its parsing, the relational engine is not able to create a plan for the query execution. To resolve the issue, the query has been modified not to use the value list variable, instead populated those values into a temp table and then use the temp table as below.

declare @sql varchar(max)
create table #temp (valuelist varchar(100))
Insert into #temp values('val1'),('val2'),('val3'),......('valhugenumber')
Select @sql = 'Select * from ' + @tablename + ' where ' + @columnname + ' in (Select valuelist from #temp)'
Exec(@sql)

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