Re: pg_restore without dropping db/table

On Thu, Mar 10, 2016 at 12:53 PM, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

> On 03/10/2016 09:41 AM, Karsten Hilbert wrote:
>
>> On Thu, Mar 10, 2016 at 10:51:05AM -0500, anj patnaik wrote:
>>
>> Does pg_restore only add new rows if I restore without deleting old db?
>>>
>>
>> No. For one thing, pg_restore cannot know what you consider
>> to be a "new row".
>>
>
> If you however do know what is new is then you might want to look at:
>
> COPY
> http://www.postgresql.org/docs/9.5/interactive/sql-copy.html
> <<IMPORTANT>>
> "COPY with a file name instructs the PostgreSQL server to directly read
> from or write to a file. The file must be accessible by the PostgreSQL user
> (the user ID the server runs as) and the name must be specified from the
> viewpoint of the server. When PROGRAM is specified, the server executes the
> given command and reads from the standard output of the program, or writes
> to the standard input of the program. The command must be specified from
> the viewpoint of the server, and be executable by the PostgreSQL user. When
> STDIN or STDOUT is specified, data is transmitted via the connection
> between the client and the server."
>
> Or you can use the psql variant \copy
>
> http://www.postgresql.org/docs/9.5/interactive/app-psql.html
>
> "Performs a frontend (client) copy. This is an operation that runs an SQL
> COPY command, but instead of the server reading or writing the specified
> file, psql reads or writes the file and routes the data between the server
> and the local file system. This means that file accessibility and
> privileges are those of the local user, not the server, and no SQL
> superuser privileges are required."
>
>
> In either case you can craft a query to limit the rows copied. The issue
> is that this assumes the both databases are static over the time you do
> this.
>
> Your other options are to use replication:
>
> http://www.postgresql.org/docs/9.5/interactive/high-availability.html
>
> or FDW
>
> https://wiki.postgresql.org/wiki/Foreign_data_wrapper
>
>
>> Best,
>> Karsten
>>
>>
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com
>
>
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>

The best way to accomplish what you want is to create a table with the same
structure in the first database as the one you want to restore to. Then you
can truncate that table, restore the data from the other db into it, and
use that to add the new rows to your table.
eg:
1. You have your original table:
CREATE TABLE orig_table
(prime_key varchar(10) ,
data_col1 integer,
data_col2 varchar(5),
CONSTRAINT orig_table_pk PRIMARY KEY (prime_key)
);
2. Duplicate table:
CREATE TABLE dup_table
(prime_key varchar(10) ,
data_col1 integer,
data_col2 varchar(5),
CONSTRAINT dup_table_pk PRIMARY KEY (prime_key)
);
3. pg_dump --format=custom -a -t orig_table second_db > your_dump.dmp
4. psql -d first_db -c truncate dup_table
5. edit your_dump.dmp and change all occurrences of orig_table to
dup_table.
6. pg_restore -a -d first_db -t dup_table your_dump.dmp
7. psql -d first_db
8. INSERT INTO orig_table
SELECT * FROM dup_table
WHERE dup.prime_key NOT IN (SELECT prime_key FROM orig_table);

--
*Melvin Davidson*
I reserve the right to fantasize. Whether or not you
wish to share my fantasy is entirely up to you.