SQL Server: Performance Insert Into vs Select Into

Question

I am hearing different things from colleagues/research. What are good guidelines in performance for, Select Into vs Insert into when creating a temp table? I know difference is minimal for small tables.

Eg: Table has 20 columns, 50 million rows.

I've had DBAs state, Insert into is faster, since compiler/parser, does not need to find Column data types on the fly. Others stating Select into is faster. We conducted performance testing, and seems select into is slightly faster.

What are good principles in figuring which is faster and why? I would think Microsoft would optimize to make insert into , just as fast, for careful programming.

Article states following.

SQL Server Performance of SELECT INTO vs INSERT INTO for temporary tables

The INSERT...INTO command will reuse data pages which are created in cache for insert/update/delete operations. It will also truncate the table when it is dropped. The SELECT...INTO command will create new pages for table creation similar to regular tables and will physically remove them when the temporary table is dropped.

Question is, why wouldn't Microsoft optimize to make insert into as fast as select into?

We have over 500 stored procedures to write for data warehouse, and require good guidelines for temp usage.

This article does not really focus on performance and reasons:

INSERT INTO vs SELECT INTO

Person in article mentioned good point:

that's mostly because SQL Server knows that there is no contention for the destination table. The performance for insert into #temp with(tablock) select * from .. is roughly the same as the performance for select * into #temp from

Answer 1

You cited two different articles that discuss two different things.

The first article compares insert..select with select into for temporary tables, and the second compares these two in general.

In general insert..select is slower because it's a fully logged operation. select into is minimally logged in the simpleand bulk logged recovery models.

The last comment you cited is about insert into with(tablock), this with(tablock) can make insert into minimally logged under some additional conditions: it should be a heap and have no indexes.

You can find the complete guide here: The Data Loading Performance Guide

It can be summarized in this table:

Note the updates for SQL Server 2016 and later in SQL Server 2016, Minimal logging and Impact of the Batchsize in bulk load operations by Parik Savjani (a Senior Program Manager with the Microsoft SQL Server Tiger team). The updated table is:

Regarding the first article. It discusses the particular case for temporary tables.

The tempdb database is special because it's always in the simple recovery model, and because logging in tempdb is different. On every server restart tempdb is recreated, this means no crash recovery is made for tempdb, and this means that logging in tempdb does not need any "after" image of modified data, only the "before" image to be able to do a rollback if there is a need. This leads to the fact that insert into..select is also minimally logged in tempdb even without tablock hint (in case of a heap that was discussed in the first article).

Conclusion "select into vs insert..select under logging aspect":

In case of a heap, insert into..select performs similarly to select into in case of temporary tables, and in general is slower when tablock hint is not used.

The second aspect is the possibility of parallel execution.

Select into can be executed in parallel starting with SQL Server 2014, and parallel insert...select was first implemented in SQL Server 2016.

I did not reproduce any performance difference between select into and insert into ..select for temporary tables on SQL Server 2012, all executed in serial.

Answer 2

What are good principles in figuring which is faster and why? I would think Microsoft would optimize to make insert into , just as fast, for careful programming.

The principles that I try to follow when analyzing something like this question are:

1. Avoid making unnecessary assumptions.
2. Read the official documentation.
3. Test the workload. The amount of testing depends on how fast I need the code to be.

I'm aware of two pieces of documentation that address your question. The first one is a blog post saying that SELECT INTO for temp tables has different behavior for eager writes as of SQL Server 2014. That is by design. So I don't think that it's correct to say that the difference is minimal for small tables. If anything, the optimization described in the blog post seems designed for smaller tables:

The change in SQL Server 2014 is to relax the need to flush these pages, as quickly, to the TEMPDB data files. When doing a select into … #tmp … or create index WITH SORT IN TEMPDB the SQL Server now recognizes this may be a short lived operation. The pages associated with such an operation may be created, loaded, queried and released in a very small window of time.

For example: You could have a stored procedure that runs in 8ms. In that stored procedure you select into … #tmp … then use the #tmp and drop it as the stored procedure completes.

Prior to the SQL Server 2014 change the select into may have written all the pages accumulated to disk. The SQL Server 2014, eager write behavior, no longer forces these pages to disk as quickly as previous versions. This behavior allows the pages to be stored in RAM (buffer pool), queried and the table dropped (removed from buffer pool and returned to free list) without ever going to disk as long memory is available. By avoiding the physical I/O when possible the performance of the TEMPDB, bulk operation is significantly increased and it reduces the impact on the I/O path resources as well.

The second piece of documentation explains that parallel insert into temp tables with INSERT INTO ... SELECT is available without a TABLOCK hint in SQL Server 2016 but requires a TABLOCK hint in SP1 and in future versions.

The issue is first fixed in SQL Server 2016 Service Pack 1 . After you apply SQL Server 2016 SP1, Parallel INSERTs in INSERT..SELECT to local temporary tables is disabled by default which reduces contention on PFS page and improves the overall performance for concurrent workload. If parallel INSERTs to local temporary tables is desired, users should use TABLOCK hint while inserting into local temporary table.

Going back to your original statement, you can't logically deduce which of the two will be faster. What is faster depends on how Microsoft designed the software and the characteristics of your workload. Making guesses about the amount of time needed to create the column definitions just isn't helpful. Testing is helpful. If your testing suggests that SELECT INTO is faster than go with that. For what it's worth, I also work on data warehouse loading with a close eye on performance and I haven't seen the difference between the two approaches be anything that's worth worrying about.

Answer 3

Every value SQL Server deals with has a datatype. The results from a SELECT are all typed. So a SELECT..INTO does not have to deduce datatypes on the fly - they are defined by the SELECT.

In contrast, with INSERT..SELECT the source and destination columns may be of different, but compatible, types. There will then be an implicit type coercion, which will consume CPU cycles. Whether the difference in execution times can be measured I couldn't say.