Introduction

As of April 2023, AWS have introduced into Redshift a new SQL command, MERGE (which originated in the SQL 2003 standard and is pretty much universal across relational databases).

This new SQL command provides an INSERT, combined with either a DELETE or an UPDATE.

The way it works is that you have your target table, which is the table you wish to modify, and a source table, which contains the rows you wish to use to modify the target table.

The MERGE command iterates over the source table, and for each row, if that row exists in the target table, that row in the target table will be either updated or deleted, and if that row does not exist in the target table, it will be inserted.

The choice of whether a row is updated or deleted is not on a per-row basis, but on a per-merge command basis; all rows which match, for a given merge command, will either be updated, or deleted.

All rows which do not match are always inserted; this is not optional (if it was, you’d just use a normal update or delete).

Finally, the source table actually has to be a table; not a sub-query or a CTE. You have to actually create and populate a table. Often this will be a staging table, so it’s not so bad, but in the cases when it is not, that’s an overhead.

Now, from a syntactical point of view, MERGE is a very nice improvement, over writing separate INSERT and UPDATE or DELETE commands.

From a performance point of view, the question obviously is whether MERGE is the same as, or better than, or worse than, separate INSERT and UPDATE or DELETE commands, and if there are any complications or gotchas to be aware of.

This question in turn in fact requires us to define what “better” and “worse” actually mean.

To my eye, there are three considerations.

Firstly, the number of new and modified blocks.

Does a MERGE produce the same, or more, or fewer, new and modified blocks, as the separate INSERT and UPDATE or DELETE queries, which would perform the same work?

Secondly, how does the work performed by the MERGE query compared to the work performed by separate INSERT and UPDATE or DELETE queries?

At the very highest level, we can look to see whether or not MERGE is a single query rather than the two for an INSERT and UPDATE or DELETE, but more meaningfully, we can look at the step plans for a MERGE, and the step plans for the equivalent INSERT and UPDATE or DELETE, and see what work is actually being done under the hood.

Thirdly, a more general question, which is what’s actually going on in general under the hood, and are there are issues we need to be aware of and look out for?

Investigation

To begin with, I create a new two node dc2.large in us-east-1 and configure the cluster like so;

set enable_result_cache_for_session to off;
set analyze_threshold_percent to 0;
set mv_enable_aqmv_for_session to false;

I then create two test tables, target_table and source_table both with one column and one row, like so;

create table target_table
(
  column_1  int8  not null encode raw distkey
)
diststyle key
compound sortkey( column_1 );

insert into
  target_table( column_1 )
values
  ( 1 );

vacuum full target_table to 100 percent;
analyze target_table;

create table source_table
(
  column_1  int8  not null encode raw distkey
)
diststyle key
compound sortkey( column_1 );

insert into
  source_table( column_1 )
values
  ( 2 );

vacuum full source_table to 100 percent;
analyze source_table;

So now we have target_table with a single row with the value 1, and source_table with a single row and the value 2.

MERGE with UPDATE

To begin with, to start getting an idea of what Redshift is going to do, I issue an EXPLAIN on the following MERGE, where I expect a single row, with the value 2, to be inserted into target_table.

explain
merge into target_table
using source_table on target_table.column_1 = source_table.column_1
when matched then update set column_1 = source_table.column_1
when not matched then insert values ( source_table.column_1 );

What I get is this;

                                QUERY PLAN                                 
---------------------------------------------------------------------------
 XN Merge Join DS_DIST_NONE  (cost=0.00..0.04 rows=1 width=8)
   Merge Cond: ("outer".column_1 = "inner".column_1)
   ->  XN Seq Scan on source_table  (cost=0.00..0.01 rows=1 width=8)
   ->  XN Seq Scan on target_table  (cost=0.00..0.01 rows=1 width=8)
 
 XN Hash Join DS_DIST_NONE  (cost=0.01..0.04 rows=1 width=14)
   Hash Cond: ("outer".column_1 = "inner".column_1)
   ->  XN Seq Scan on target_table  (cost=0.00..0.01 rows=1 width=14)
   ->  XN Hash  (cost=0.01..0.01 rows=1 width=8)
         ->  XN Seq Scan on source_table  (cost=0.00..0.01 rows=1 width=8)
 
 XN Seq Scan on merge_tt_5f9f1e92e6878  (cost=0.00..0.01 rows=1 width=8)
(12 rows)

Now that’s a pretty unusual query plan. It looks like three separate queries.

Looking at the query history, what I see is quite surprising and wholly novel.

Now, the query history page is pretty wide - there’s lots of columns - so the first screenshot is the whole page (which requires 50% zoom), but the second is at 100% zoom and moved over to the right so we can see the column showing the first 48 characters of query text.

The order of queries is oldest at the bottom. The EXPLAIN command is highlighted, and is at the bottom, and we can its text - it begins with “explain\n”.

To my considerable surprise, the EXPLAIN has created a temp table and analyzed it (the analyze leads to the final command, the padb_fetch_sample).

This is new - I’ve never seen an EXPLAIN do actual work before, and this is interesting, because it means if you ran this EXPLAIN on a serious table, a Big Data table, you might find its doing a lot of work, and temp table persist until a session ends, so every EXPLAIN you issue is going to consume additional disk space (until your session ends).

The next question then is what is in the temp table.

To find this out, we look at the text of the CREATE TEMP TABLE command.

The text has been issued as a single line of text with no newlines, so I clearly need to add an option to format the text - as it is, I have copy-and-pasted the text, and manually formatted the text, below;

/* MERGE REWRITTEN */
CREATE TEMP TABLE
  merge_tt_5f9f2b11d9736
AS
  (
    SELECT
      "public"."source_table"."column_1" AS "column_1"
    FROM
      "public"."source_table"
    WHERE
      NOT EXISTS
      (
        SELECT
          CAST(1 AS INT4)
        FROM
          "public"."target_table"
        WHERE
          "public"."target_table"."column_1" = "public"."source_table"."column_1"
      )
  )

Wowsers. Three queries already - the create temp itself, and two sub-queries.

Looking at the code, what the temp table consists of is all rows which are in source_table but not in target_table - in other words, all the rows which would be inserted by the merge.

So, indeed - if this was done on tables of any size, that could perfectly well be a lot of rows.

To my surprise again however, the temp tables being produced appear to be silently deleted immediately after the EXPLAIN. As far as I can tell, they are not extant after the query, but I can see no DROP TABLE, and I’m still in the same session.

The temp table has no specified encodings, so Redshift is using its default encodings, which are really no good at all, and also auto for distribution and sorting. This is an inefficient table. At least where it’s a temp table, it does not participate in k-safety, which will make it faster, and where it’s a single insert into a new table, the rows will be sorted (but since it’s an auto table, and the table is new, it will start as ALL, then if enough data is inserted become EVEN (I don’t know when the redistribution happens - I never use AUTO - I suspect it might be immediately after the query completes, so the table could then having been created, in effect be created again, as a new copy has to be made with the new distribution style), and both ALL and EVEN preclude merge joins. I’ve no idea what sort key AUTO will choose for a brand new table - unsorted, perhaps? Redshift in principle can change this, but I believe this takes some time, Redshift has to build up some history of queries on the table, so I do not expect it to change from whatever the initial default is.)

Let’s try now actually executing the MERGE, and let’s see what we get.

Well well well. Isn’t this interesting.

To make it easier to see, and to show this is a multi-query transaction, I’ve brought up the transaction which holds the merge.

The transaction has queries listed from top to bottom as first to last in the transaction.

What we see are;

1. CREATE TEMP TABLE (all rows which would be inserted).
2. ANALYZE of the temp table.
3. The padb_fetch_sample for the analyze.
4. An actual explicit UPDATE command.
5. A MERGE command.
6. The final commit.

Now, with regard to the temp table, the work being done in the query is being done because SELECT in Redshift does not support the ALL argument to EXCEPT.

If it did, you’d produce the set of rows by excepting all from source_table all rows in target_table, which is sweet, simple and easy to read, but without ALL, you get one row only of each distinct row, regardless of how many of that row exist in source_table, which is not what you want.

In the first version of this document, I did not understand why the temp table was in use. After publication, a Slack user by the name of mauro explained it to me, and so I can now explain it to you.

The reason for the temp table is that the specification of the MERGE command mandates each source row either matches or does not match, but never both; an UPDATE and INSERT pair without a temp table can allow a source row to match and not match - this happens when the source row matches, but the update command that is then as such issued converts the row into a row which no longer matches, and then of course the INSERT, reading the source table a second time, will go right ahead and insert.

In merges where the UPDATE does not cause rows to match, the temp table is indeed unnecessary, so the implementation of MERGE in Redshift is a kind of worst-case implementation - it has to be like that, to actually meet the specification of MERGE, but whenever the merge you’re issuing would not cause matched rows to no longer match, the temp table is not needed, and then it’s pure overhead.

So now we a solid overview of what’s going on; MERGE is not a new command as such, but a macro.

We can now though look at the query texts, and the step plans, of the queries, to see what they’re doing.

The CREATE TEMP TABLE turns out not surprisingly to be identical to that emitted by the EXPLAIN command.

The ANALYZE is just doing what ANALYZE does, so that’s not of particular interest to us.

The UPDATE is interesting, and we see this, which again is a single line of text, which I have taken out and formatted for easy reading;

/* MERGE REWRITTEN */
UPDATE
  "public"."target_table"
SET
  "column_1" = "public"."source_table"."column_1"
FROM
  "public"."source_table"
WHERE
  "public"."target_table"."column_1" = "public"."source_table"."column_1"

This does what we’d expect; it updates every row in target_table which can be found in source_table.

Now we come to the mysterious MERGE.

As we can see, the text is exactly that of the original MERGE command, which tells us nothing.

However, we know we’re missing the insert, so my guess is what’s really going on here is the insert - and we can check by looking at the step plan.

Bingo.

The four highlighted lines are the insert step (one line per slice).

The actual SQL for this query must be that of an INSERT, but my guess is it is being replaced by the MERGE SQL by Redshift, perhaps so people looking at the query history can actually see the command they issued.

What can I say? on one hand, I do want to see the actual commands being issued, on the other, I also want to see what’s actually going on. System table design needs to be improved, rather than having stuff shoe-horned in.

MERGE with DELETE

So, we start from scratch; we drop the tables we made, and re-create them, exactly as we did for merge with update.

We then issue the following command;

merge into target_table
using source_table on target_table.column_1 = source_table.column_1
when matched then delete
when not matched then insert values ( source_table.column_1 );

As before, the screenshot of the query history for the MERGE, and the close up of the query texts;

The text of the CREATE TEMP TABLE command is unchanged, which populates the temp table with all the rows which would be inserted, and we can see the UPDATE has been replaced by DELETE (the highlighted line).

Finally, we have the text of the DELETE;

/* MERGE REWRITTEN */
DELETE FROM
  "public"."target_table"
USING
  "public"."source_table"
WHERE
  "public"."target_table"."column_1" = "public"."source_table"."column_1"

As expected, here we see that any rows which are present in source_table and also present in target_table are now deleted, and the finally we have the disguised INSERT, which puts all rows from the temp table into target_table.

Benchmarks

The benchmarks were made before I understood what the temp table was for, and were intended to show the performance penalty of the temp table.

As it is, the temp table is by no means always necessary, and so the benchmarks are still useful in that they show the overhead of using the temp table (which MERGE will always use) against the situation where you issue an UPDATE and INSERT yourself, without the temp table.

The test method is to create target_table which has two columns, the first always holding the same value and being the distribution key (so all work is on a single slice), the second being an auto-incrementing integer but which we can explicitly over-ride.

We use the auto-increment functionality to generate 3,263,442 rows which each have a unique value.

We then create source_table in exactly the same way, but over-ride auto-increment to add 3,263,442 rows with the same value.

Both tables are then fully vacuumed and analyzed.

We then perform a MERGE, timing the result, and then repeat the setup, but next performing a transaction with an UPDATE and then an INSERT, and we sum the times for those two queries.

All times are taken using \timing in the psql client, so they include network time, but I expect the differences to be large, so I’m not worried about this.

The SQL for all this is given in Appendix A : Full Setup and Test SQL.

So, after all our setup work, we have target_table with 3,263,442 rows of unique integers, and source_table with 3,263,442 rows of the same unique integers, and also 3,263,442 rows of the value 999,999,999.

All rows are on a single slice, to help simplify what’s going on, and all encodings are raw.

This is the MERGE query;

merge into target_table
using source_table on target_table.column_2 = source_table.column_2
when matched then update set column_2 = source_table.column_2
when not matched then insert values ( 1, source_table.column_2 );

And these are the UPDATE + INSERT queries, which are identical to those emitted by MERGE, except I’m not using the temporary table;

begin;

update
  target_table
set
  column_2 = source_table.column_2
from
  source_table
where
  target_table.column_2 = source_table.column_2;

insert into
  target_table( column_1, column_2 )
select
  1,
  source_table.column_2
from
  source_table
where
  not exists
  (
    select
      cast( 1 as int4 )
    from
      target_table
    where
      target_table.column_2 = source_table.column_2
  );

commit;

So, as noted, my manual version doesn’t bother with the temp table, because there’s no need for it.

After the UPDATE + INSERT has run, we can confirm target_table has been updated, as follows;

dev=# select count(*) from target_table;
  count  
---------
 6526884
(1 row)

dev=# select count(*) from source_table;
  count  
---------
 6526884
(1 row)

dev=# select sum(column_2) from target_table;
       sum        
------------------
 3284742091057521
(1 row)

dev=# select sum(column_2) from source_table;
       sum        
------------------
 3284742091057521
(1 row)

dev=# select
  count(*)
from
  (
    select
      column_1,
      column_2
    from
      target_table

    except

    select
      column_1,
      column_2
    from
      source_table
  );
 count 
-------
     0
(1 row)

Using the code in Appendix A, I ran the full table setup code and the test code for MERGE, and the full table setup code and the test code for UPDATE + INSERT tests five times each (both after initial runs to get query compilation out of the way).

Discarding the slowest and fastest result, mean is 3.34s, standard deviation is 0.07s.

Discarding the slowest and fastest result, mean is 2.73s, standard deviation is 0.14s.

The results are as expected; the queries are the same, except the UPDATE and INSERT are not using a temp table, so they’re doing less work.

Finally, note that UPDATE in its FROM can use a view or sub-query, where-as MERGE in its USING cannot - it can use only a table. This on the face of it is a wholly novel restriction imposed by MATCH, and I can’t see a need or reason for it.

Summary

So where does this leave us?

Well, MERGE does the following when it is updating;

1. First, you must create and populate the source table, as the source must be a table, not a sub-query or view.
2. Scan the source table, and the target table, inserting all rows which are not present into a temp table.
3. Scan the source table, and the target table, updating all rows in the target table which are present in the source table.
4. Insert into the target table all rows in the temp table.

(Why the source table in MERGE must be a table rather than a sub-query or view, I do not know, as a temp table is made from it anyway, and UPDATE is fine with both sub-queries and views.)

Now, if you did this manually, you would do the following;

1. Scan the source table, and the target table, updating all rows in the target table which are present in the source table.
2. Scan the source table, and the target table, inserting all rows which are not present in the target table.

As you can see, manual steps 1 and 2 are MERGE steps 2 and 3, except that with MERGE, the insert work goes via a temporary table, and also when performed manually, the source table can be a sub-query or view, unlike when using MATCH, where you must use an actual table, which means creating and populating that table.

In short, MERGE is more expensive, as it does everything you would do manually, plus it requires you to create the source table as an actual table, and it writes the rows to be inserted to a temp table (which given how the temp table is created, prevents merge joins).

MERGE is not really a new command, but is implemented using using existing SQL commands and functionality, and on the face of it, you can do a better job than MERGE does.

MERGE also brings a new and seemingly unnecessary restriction, in that it’s USING must use a table and cannot use a sub-query or view, unlike the FROM on UPDATE.

My feeling, although I’ve not thought it through in any details, is that there could be opportunities here for optimization, if MERGE has been written as a genuinely new command - I could imagine it making a single pass over the target table, and in a single query, performing the insert and the update or delete.

However, I’m of the view the core code base of Redshift can no longer be meaningfully modified - a symptom of this being that new functionality is being bolted on the outside, rather than adding to or modifying the internals of Redshift - and the implementation of MERGE is in line with that theory.

Detailed answers to the full set of questions posited in the Introduction do not need to be investigated further, because MERGE should not be used, so the answers are irrelevant.

There’s also here a larger picture which needs to be considered.

Redshift is a sorted clustered relational database. Sorting is a computing method which offers with no hardware costs staggering efficiency - and so, scaling - but when and only when operated correctly. If operated incorrectly, it gives you nothing; the database internally is having to doing exactly the same work as an unsorted database, you will only be able to scale by adding hardware, and you do not have indexes. You would be better off being on an unsorted clustered relational database, as you can still scale by hardware, but you get indexes (and usually a great deal more functionality than Redshift offers).

Updates - which is to say, upserts, which is to say, merge - broadly speaking usually mean sorting is being operated incorrectly. If a system is using upserts such that merge looks interesting, probably you should not be on Redshift; and so in that sense, to my eye, even if merge was an improvement over manually issuing insert and a delete or update, it still wouldn’t make sense - it is improper functionality to optimize on a sorted relational database.

Moreover, if we look at the work the manual version of MERGE has to do because of the lack of support for the ALL argument to SELECT EXCEPT, what actually might have improved performance is implementing ALL for EXCEPT. That would have been a genuine and useful enhancement.

Why was time spent producing MERGE, which is slower and more restricted than a manual UPDATE and INSERT, than producing an improved EXCEPT, which would have improved a manual UPDATE and INSERT?

As it is, given the internal implementation of MERGE, this looks to my eye like a marketing exercise, but with negative technical value. If you use it, you think you’re doing well and you’ve got something new and improved, when it fact you’re being harmed by it.

If MERGE was modified not to use the temp table, and removed the restriction where the source table must be a table, then it would in performance terms be identical to a manual UPDATE + INSERT, but you would now have the nice syntax of MERGE, which would be the sole gain. This could also have some use in porting existing SQL from other systems to Redshift.

The only caveat in all this is that I still do not understand why a temp table is being used. I can see no reason for it at all, which worries me; have I missed something? on the other hand, I can correctly duplicate MERGE without it, so it really does look to be unnecessary.

Conclusion

The MERGE command is implemented as a wrapper around existing SQL commands, calling CREATE TEMP TABLE, UPDATE and INSERT or DELETE.

You can yourself manually issue exactly the SQL commands MERGE issues; it is not an implementation which brings together the work of a merge into a single, optimized command, but rather, it is a macro.

The SQL standard specification of the MERGE command mandates each source row either matches or does not match, but never both; an UPDATE and INSERT pair without a temp table can allow a source row to match and not match - this happens when the source row matches, but the update command that is then as such issued converts the row into a row which no longer matches, and then of course the INSERT, reading the source table a second time, will go right ahead and insert.

In merges where the UPDATE does not cause rows to match, the temp table is indeed unnecessary, so the implementation of MERGE in Redshift is a kind of worst-case implementation - it has to be like that, to actually meet the specification of MERGE, but whenever the merge you’re issuing would not cause matched rows to no longer match, the temp table is not needed, and then it’s pure overhead.

As such, in the benchmarks (five iterations, slowest and fastest discarded), we see;

Additionally, MERGE mandates the use of and only of a table for the merge source rows, which is not necessary when issuing an UPDATE yourself, as UPDATE works also with sub-queries and views.

All in all, the MERGE command is syntactically very pleasant, but under the hood, the implementation in Redshift is I think absolutely not what people expected.

Finally, MERGE in Redshift (but not in Postgres, for example) restricts itself to a table for source rows, and I can’t see any reason for that. The SQL emitted by the MERGE command obfuscates its INSERT, so I can’t know what SQL it emits for that, but testing the CREATE TEMP TABLE and the UPDATE, and my own INSERT, I was able to use a sub-query in all cases.

Credits

1. mauro

   In version 1 of this document, I did not understand the purpose of the temp table being used by MERGE (where I work primarily on Redshift, I never use UPDATE, never upsert, and MERGE was new to me). The explanation was provided by mauro, which is that the specification of the MERGE command mandates each source row either matches or does not match, but never both; an UPDATE and INSERT pair without a temp table can allow a source row to match and not match - this happens when the source row matches, but the update command that is then as such issued converts the row into a row which no longer matches, and then of course the INSERT, reading the source table a second time, will go right ahead and insert.

   In merges where the UPDATE does not cause rows to match, the temp table is indeed unnecessary, so the implementation of MERGE in Redshift is a kind of worst-case implementation - it has to be like that, to actually meet the specification of MERGE, but whenever the merge you’re issuing would not cause matched rows to no longer match, the temp table is not needed, and then it’s pure overhead.

Revision History

v1

• Initial release.

v2

• Updated with explanation from mauro about the use by MERGE of its temp table.

v3

• Added “About the Author”. made site name in title a link, and made each chapter start a new page.

Appendix A : Full Setup and Test SQL

Off-One Setup SQL

First, the one-off configuration (all work performed in psql).

set enable_result_cache_for_session to off;
set analyze_threshold_percent to 0;
set mv_enable_aqmv_for_session to false;
\timing on

Target and Source Table Setup SQL

Next, the SQL which creates and populates target_table and source_table.

drop table if exists target_table;

create table target_table
(
  column_1  int2  not null encode raw distkey,
  column_2  int8  not null encode raw generated by default as identity( 1, 1 )
)
diststyle key
compound sortkey( column_1, column_2 );

insert into
  target_table( column_1 )
values
  ( 1 );

insert into
  target_table( column_1 )
select
  1
from
  target_table as t1,
  target_table as t2;

insert into
  target_table( column_1 )
select
  1
from
  target_table as t1,
  target_table as t2;

insert into
  target_table( column_1 )
select
  1
from
  target_table as t1,
  target_table as t2;

insert into
  target_table( column_1 )
select
  1
from
  target_table as t1,
  target_table as t2;

insert into
  target_table( column_1 )
select
  1
from
  target_table as t1,
  target_table as t2;

vacuum full target_table to 100 percent;
analyze target_table;

/* MG2 : Now, examining the table, we see this;

         dev=# select * from target_table order by column_1, column_2 limit 10;
          column_1 | column_2 
         ----------+----------
                 1 |        1
                 1 |        2
                 1 |        6
                 1 |       10
                 1 |       14
                 1 |       18
                 1 |       22
                 1 |       26
                 1 |       30
                 1 |       34
         (10 rows)
*/

drop table if exists source_table;

create table source_table
(
  column_1  int8  not null encode raw distkey,
  column_2  int8  not null encode raw generated by default as identity( 1, 1 )
)
diststyle key
compound sortkey( column_1, column_2 );

insert into
  source_table( column_1 )
values
  ( 1 );

insert into
  source_table( column_1 )
select
  1
from
  source_table as t1,
  source_table as t2;

insert into
  source_table( column_1 )
select
  1
from
  source_table as t1,
  source_table as t2;

insert into
  source_table( column_1 )
select
  1
from
  source_table as t1,
  source_table as t2;

insert into
  source_table( column_1 )
select
  1
from
  source_table as t1,
  source_table as t2;

insert into
  source_table( column_1 )
select
  1
from
  source_table as t1,
  source_table as t2;

insert into
  source_table( column_1, column_2 )
values
  ( 1, 999999999 );

insert into
  source_table( column_1, column_2 )
select
  1, 999999999
from
  source_table as t1,
  source_table as t2
where
      t1.column_2 = 999999999
  and t2.column_2 = 999999999;

insert into
  source_table( column_1, column_2 )
select
  1, 999999999
from
  source_table as t1,
  source_table as t2
where
      t1.column_2 = 999999999
  and t2.column_2 = 999999999;

insert into
  source_table( column_1, column_2 )
select
  1, 999999999
from
  source_table as t1,
  source_table as t2
where
      t1.column_2 = 999999999
  and t2.column_2 = 999999999;

insert into
  source_table( column_1, column_2 )
select
  1, 999999999
from
  source_table as t1,
  source_table as t2
where
      t1.column_2 = 999999999
  and t2.column_2 = 999999999;

insert into
  source_table( column_1, column_2 )
select
  1, 999999999
from
  source_table as t1,
  source_table as t2
where
      t1.column_2 = 999999999
  and t2.column_2 = 999999999;

vacuum full source_table to 100 percent;
analyze source_table;

/* MG2 : if we now examine source_table, we find;

         dev=# select count(*) from source_table;
           count  
         ---------
          6526884
         (1 row)

         (which is 2*3,263,442 = 6,526,884)

         dev=# select count(*) from source_table where column_2 = 999999999;
           count  
         ---------
          3263442
         (1 row)

         dev=# select count(*) from source_table where column_2 != 999999999;
           count  
         ---------
          3263442
         (1 row)

         dev=# select * from target_table order by column_1, column_2 limit 10;
          column_1 | column_2 
         ----------+----------
                 1 |        1
                 1 |        2
                 1 |        6
                 1 |       10
                 1 |       14
                 1 |       18
                 1 |       22
                 1 |       26
                 1 |       30
                 1 |       34
         (10 rows)

         dev=# select count(*) from source_table where source_table.column_2 in ( select column_2 from target_table );
           count  
         ---------
          3263442
         (1 row)
*/

MERGE Test SQL

merge into target_table
using source_table on target_table.column_2 = source_table.column_2
when matched then update set column_2 = source_table.column_2
when not matched then insert values ( 1, source_table.column_2 );

UPDATE + INSERT Test SQL

begin;

update
  target_table
set
  column_2 = source_table.column_2
from
  source_table
where
  target_table.column_2 = source_table.column_2;

insert into
  target_table( column_1, column_2 )
select
  1,
  source_table.column_2
from
  source_table
where
  not exists
  (
    select
      cast( 1 as int4 )
    from
      target_table
    where
      target_table.column_2 = source_table.column_2
  );

commit;

About the Author

I am a C programmer - kernel development, high performance computing, networking, data structures and so on.

I read the C. J. Date book, the classic text on relational database theory, and having learned the principles, wrote a relational database from scratch in C, which purely by chance set me up quite nicely for what came next, moving into data engineering in late 2011, when I joined as the back-end engineer two friends in their startup.

In that startup, I began using Redshift the day it came out, in 2012 (we had been trying to get into the beta programme).

We were early, heavy users for a year and a half, and I ending up having monthly one-to-one meetings with one of the Redshift team managers, where one or two features which are in Redshift today originate from suggestions made in those meetings, such as the distribution style ALL.

Once that was done, after a couple of years of non-Redshift data engineering work, I returned to Redshift work, and then in about mid-2018 contracted with a publisher to write a book about Redshift.

The book was largely written but it became apparent I wanted to do a lot of things which couldn’t be done with a book - republish on every new Redshift release, for example - and so in the end I stepped back from the contract and developed the web-site, where I publish investigation into, and ongoing monitoring of, Redshift.

So for many years now I’ve been investigating Redshift sub-systems full-time, one by one, and this site and these investigations are as far as I know the and the only source of this kind of information about Redshift.

Redshift Cluster Cost Reduction Service

I provide consultancy services for Redshift - advice, design, training, getting failing systems back on their feet pronto, the usual gamut - but in particular offer a Redshift cluster cost reduction service, where the fee is and only is one month of the savings made.

Broadly speaking, to give guidance, savings are expected fall into one of two categories; either something like 20%, or something like 80%. The former is for systems where the business use case is such that Redshift cannot be operated correctly, and this outcome requires no fundamental re-engineering work, the latter is for systems where Redshift can be operated correctly, and usually requires fundamental re-engineering work (which you may or may not wish to engage in, despite the cost savings, in which case we’re back to the 20%).

Details and contact information are on the web-site.