I have two tables: table1 with 61 million rows and table2 with 59 millions rows. The columns in both are the same (name and type). Both are imported from backup files.

I want to merge these two tables into one table keeping only unique records.

For example, in table1 I have the following records:

NAME    CODE   ...
Name1   001    ...
Name2   002    ...
Name3   003    ...
Name4   004    ...

And in table2:

NAME    CODE   ...
Name1   001    ...
Name2   002    ...
Name5   005    ...
Name2   002    ...

The result table should be something like this:

NAME    CODE   ...
Name1   001    ...
Name2   002    ...
Name3   003    ...
Name4   004    ...
Name5   005    ...

Edit to provide more info as requested by @Erwin Brandstetter:

  • table1 has 15 GB. I am inserting records on table2 but when I finish it will have almost the same size.

  • I have more than 300 GB of free space (and an external HD if needed).

  • Total RAM is 8 GB.

  • No indexes (i can create if needed).

  • No concurrent access (it is accessed just by me on local machine).

  • I can create a new, third table if needed.

  • Here if the full table definition:

   nb double precision,
   nome character varying(200),
   nasc double precision,
   cpf double precision,
   especie double precision,
   dib double precision,
   valor double precision,
   banco_pagt double precision,
   ag_banco double precision,
   orgao_pag double precision,
   aps double precision,
   meio_pagto double precision,
   banco_empr double precision,
   contrato character varying(200),
   vl_empres double precision,
   comp_ini_d double precision,
   parcelas double precision,
   vl_parcela double precision,
   tipo_empre double precision,
   endereco character varying(200),
   bairro character varying(200),
   municipio character varying(200),
   uf character varying(2),
   cep double precision,
   sit_empres double precision,
   dt_averb double precision,
   dt_exc double precision,
   id double precision

The id column is filled with a row number, so it could be used to perform batch operations if the full operation becomes too slow.
All columns except the id column should be considered for the unique check.

  • The basic problem is simple, but the best solution depends on details you did not provide. Size of the tables / available disk space? Available RAM? Complete table definition showing data types, constraints and indexes? Is there concurrent access, and if yes, what's the situation exactly? Can you afford to create a new table or does one of the two need to stay in place? (Due to concurrent access, dependencies, etc.) Is performance important? Commented Nov 4, 2015 at 13:10
  • How should those columns be treated that you have hidden from use (the ones indicated by ...)? What do you want to do if there are rows with the same combination of name and code in both tables. Which values for the other columns should be used?
    – user1822
    Commented Nov 4, 2015 at 13:18
  • @ErwinBrandstetter and i edited to provide more details
    – Ewerton
    Commented Nov 4, 2015 at 13:33
  • Do you need to preserve the id column at all? Or can we just add a new serial number? Commented Nov 4, 2015 at 13:36
  • I dont need to preserve the ID column, other columns could be created if needed
    – Ewerton
    Commented Nov 4, 2015 at 13:38

2 Answers 2


The basic problem can be solved with various simple queries. Considering all columns:

TABLE tbl1

Given this additional information:

All columns except the id column should be considered for the unique check.


I don't need to preserve the ID column.

Just drop the id column, then you can proceed with the simple query above.

I would import to temporary tables (much faster, less overhead) and only write the final result (tbl3) to a regular table - in one session because temporary tables are dropped automatically at the end of the session.

CREATE TEMP TABLE tbl1 ( <columns from above, without id> );
COPY tbl1 FROM '/path/to/file1';

CREATE TEMP TABLE tbl2 ( <columns from above, without id> );
COPY tbl2 FROM '/path/to/file2';

Alternatively, to preserve the input tables across sessions, you could use unlogged tables.

For best performance create and fill the target with CREATE TABLE AS and add the PK constraint in the same transaction:


CREATE SEQUENCE tbl3_tbl3_id_seq;

SELECT nextval('tbl3_tbl3_id_seq'::regclass)::int AS tbl3_id, *
FROM  (TABLE tbl1 UNION TABLE tbl2 ) sub;

   ADD CONSTRAINT tbl3_pkey PRIMARY KEY(tbl3_id)
 , ALTER COLUMN tbl3_id SET DEFAULT nextval('tbl3_tbl3_id_seq'::regclass);

ALTER SEQUENCE tbl3_tbl3_id_seq OWNED BY tbl3.tbl3_id;    


Replace all occurrences of "tbl3" with our desired table name.

Detailed explanation in this related answer:

I added a serial column (tbl3_id) as surrogate PK to the target table. Adding the actual PK constraint at the end (of the same session) is the fastest way.


Before you do it, test whether double precision is the best data type for all those columns. Chances are, some of them could be integer (cheaper for whole numbers) or must really be numeric (loss-less). If so, adapt your temp tables to begin with.

  • I think it's not as easy as that. Unfortunately Ewerton didn't tell us how exactly those columns indicated by .... should be treated. I suspect those should not be considered for the unique check.
    – user1822
    Commented Nov 4, 2015 at 13:19
  • @a_horse_with_no_name: Probably right. So far, the OP only mentioned "unique records". The answer fits the question. Commented Nov 4, 2015 at 13:21
  • All columns, except the ID, should be considered for unique check, i edited the question
    – Ewerton
    Commented Nov 4, 2015 at 13:34
  • Sorry @ErwinBrandstetter but i dont understand yet, the operation consists in two steps? First create the temp tables (tbl1, and tbl2) then execute the union script? if so, what i need to replace in te '/path/to/file1' ?
    – Ewerton
    Commented Nov 4, 2015 at 15:30
  • 1
    I think i understand now. I Already have the two tables imported, so just the union script is enough for my case. Thanks, it was a master answer.
    – Ewerton
    Commented Nov 4, 2015 at 17:34

If all columns except the ID must be unique and the ID does not need to be preserved the way to go could be:

  1. Create the new empty table as table1. Make the ID an 'auto-increment'.
  2. Insert a union of table1 and table2 with all fields except the ID and insert these in the new table.

This way you enter unique rows and the ID is generated by the database.

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