What's the best way to copy a subset of a table's rows from one database to another in Postgres?

Question

I\'ve got a production DB with, say, ten million rows. I\'d like to extract the 10,000 or so rows from the past hour off of production and copy them to my local box. How do I do

Answer 1

From within psql, you just use copy with the query you gave us, exporting this as a CSV (or whatever format), switch database with \c and import it.

Look into \h copy in psql.

Answer 2

source server:

BEGIN;

CREATE TEMP TABLE mmm_your_table_here AS
    SELECT * FROM your_table_here WHERE your_condition_here;

COPY mmm_your_table_here TO 'u:\\source.copy';

ROLLBACK;

your local box:

-- your_destination_table_here must be created first on your box

COPY your_destination_table_here FROM 'u:\\source.copy';

article: http://www.postgresql.org/docs/8.1/static/sql-copy.html

Answer 3

Source:

psql -c "COPY (SELECT * FROM mytable WHERE ...) TO STDOUT" > mytable.copy

Destination:

psql -c "COPY mytable FROM STDIN" < mytable.copy

This assumes mytable has the same schema and column order in both the source and destination. If this isn't the case, you could try STDOUT CSV HEADER and STDIN CSV HEADER instead of STDOUT and STDIN, but I haven't tried it.

If you have any custom triggers on mytable, you may need to disable them on import:

psql -c "ALTER TABLE mytable DISABLE TRIGGER USER; \
         COPY mytable FROM STDIN; \
         ALTER TABLE mytable ENABLE TRIGGER USER" < mytable.copy

Answer 4

With the constraint you added (not being superuser), I do not find a pure-SQL solution. But doing it in your favorite language is quite simple. You open a connection to the "old" database, another one to the new database, you SELECT in one and INSERT in the other. Here is a tested-and-working solution in Python.

 #!/usr/bin/python

""" 

Copy a *part* of a database to another one. See
<http://stackoverflow.com/questions/414849/whats-the-best-way-to-copy-a-subset-of-a-tables-rows-from-one-database-to-anoth>

With PostgreSQL, the only pure-SQL solution is to use COPY, which is
not available to the ordinary user.

Stephane Bortzmeyer <bortzmeyer@nic.fr>

"""

table_name = "Tests"
# List here the columns you want to copy. Yes, "*" would be simpler
# but also more brittle.
names = ["id", "uuid", "date", "domain", "broken", "spf"]
constraint = "date > '2009-01-01'"

import psycopg2

old_db = psycopg2.connect("dbname=dnswitness-spf")
new_db = psycopg2.connect("dbname=essais")
old_cursor = old_db.cursor()
old_cursor.execute("""SET TRANSACTION READ ONLY""") # Security
new_cursor = new_db.cursor()
old_cursor.execute("""SELECT %s FROM %s WHERE %s """ % \
                       (",".join(names), table_name, constraint))
print "%i rows retrieved" % old_cursor.rowcount
new_cursor.execute("""BEGIN""")
placeholders = []
namesandvalues = {}
for name in names:
    placeholders.append("%%(%s)s" % name)
for row in old_cursor.fetchall():
    i = 0
    for name in names:
        namesandvalues[name] = row[i]
        i = i + 1
    command = "INSERT INTO %s (%s) VALUES (%s)" % \
              (table_name, ",".join(names), ",".join(placeholders))
    new_cursor.execute(command, namesandvalues)
new_cursor.execute("""COMMIT""")
old_cursor.close()
new_cursor.close()
old_db.close()
new_db.close()