single quotes appear arround value after running c

问题:

I am new to postgres and I play around with data loading. Here is the table definition from postgres 9.2 spec:

CREATE TABLE weather (
    city varchar(80),
    temp_lo int, -- low temperature
    temp_hi int, -- high temperature
    prcp real, -- precipitation
    date date
 );

I prepared the following data file (weather.txt):

San Francisco   43  57  0.0 '1994-11-29'
Hayward 54  37  0.0 '1994-11-29'

and ran the COPY command:

COPY weather FROM '~aviad/postsgres/playground/weather.txt';

now, when I run select * from weather; I see that single quotes appears around the city values. This does not happen when I run simple INSERT e.g:

INSERT INTO weather VALUES ('San Francisco', 46, 50, 0.25, '1994-11-27');

I wonder:

1. what is the reason for wrapping text values by single quotes?
2. What is the correct way to put the text data in the file used by COPY to avoid single quote wrapping?

回答1:

What you describe in your question is obviously not what's really happening. COPY would fail trying to import string literals with redundant single quotes into a date column.

To get rid of redundant quotes, import to a temporary table with text column, then INSERT INTO the target table trimming the quotes:

CREATE TEMP TABLE wtmp (
   city text
 , temp_lo int
 , temp_hi int
 , prcp real
 , date text  -- note how I use text here.
);

COPY wtmp FROM '~aviad/postsgres/playground/weather.txt';

INSERT INTO weather (city, temp_lo, temp_hi, prcp, date)
SELECT city, temp_lo, temp_hi, prcp, trim(date, '''')::date
FROM   wtmp
-- ORDER BY ?
;

The temp table is dropped automatically at the end of your session.

Reserved words as identifiers

I see you copied the example from the manual. Here is the deep link to the current manual.

While being correct, that example in the manual is unfortunate. I'd advise not to use reserved words like date as column names. As you can see here date is a reserved word in every SQL standard. It's allowed to use in Postgres and I can see how it's tempting for a simple example. But that doesn't make it a good idea. Generally, you should be in the habit of avoiding reserved words as identifiers. It leads to confusing error messages and needlessly incompatible SQL code.

回答2:

1 - The practice of wrapping text in quoted identifiers is done in the event that the demiliter used within the file (comma in your case) also appears within the text string

2 - I do not know must about postgres, but if you specifiy the quoted identifier in your COPY command, it should remove it during the import:

COPY weather FROM '~aviad/postsgres/playground/weather.txt' (QUOTE '?');

something along those lines. Simply try replacing the ? with the quoted identifier - in your case, I would try this first:

COPY weather FROM '~aviad/postsgres/playground/weather.txt' (QUOTE '''');

You also might want to check out: http://www.postgresql.org/docs/9.2/static/sql-copy.html as there are many different switches you can use in the COPY command