How to ignore errors with psql \copy meta-command

Question

I am using psql with a PostgreSQL database and the following copy command:

\COPY isa (np1, np2, sentence) FROM 'c:\Downloads\isa.txt' WITH DELIMITER '|'

I get:

ERROR:  extra data after last expected column

How can I skip the lines with errors?

Answer 1

You cannot skip the errors without skipping the whole command up to and including Postgres 14. There is currently no more sophisticated error handling.

\copy is just a wrapper around SQL COPY that channels results through psql. The manual for COPY:

COPY stops operation at the first error. This should not lead to problems in the event of a COPY TO, but the target table will already have received earlier rows in a COPY FROM. These rows will not be visible or accessible, but they still occupy disk space. This might amount to a considerable amount of wasted disk space if the failure happened well into a large copy operation. You might wish to invoke VACUUM to recover the wasted space.

Bold emphasis mine. And:

COPY FROM will raise an error if any line of the input file contains more or fewer columns than are expected.

COPY is an extremely fast way to import / export data. Sophisticated checks and error handling would slow it down.

There was an attempt to add error logging to COPY in Postgres 9.0 but it was never committed.

Solution

Fix your input file instead.

If you have one or more additional columns in your input file and the file is otherwise consistent, you might add dummy columns to your table isa and drop those afterwards. Or (cleaner with production tables) import to a temporary staging table and INSERT selected columns (or expressions) to your target table isa from there.

Related answers with detailed instructions:

• How to update selected rows with values from a CSV file in Postgres?
• COPY command: copy only specific columns from csv

Answer 2

It is too bad that in 25 years Postgres doesn't have -ignore-errors flag or option for COPY command. In this era of BigData you get a lot of dirty records and it can be very costly for the project to fix every outlier.

I had to make a work-around this way:

1. Copy the original table and call it dummy_original_table
2. in the original table, create a trigger like this:

    CREATE OR REPLACE FUNCTION on_insert_in_original_table() RETURNS trigger AS  $$  
    DECLARE
        v_rec   RECORD;
    BEGIN
        -- we use the trigger to prevent 'duplicate index' error by returning NULL on duplicates
        SELECT * FROM original_table WHERE primary_key=NEW.primary_key INTO v_rec;
        IF v_rec IS NOT NULL THEN
            RETURN NULL;
        END IF; 
        BEGIN 
            INSERT INTO original_table(datum,primary_key) VALUES(NEW.datum,NEW.primary_key)
                ON CONFLICT DO NOTHING;
        EXCEPTION
            WHEN OTHERS THEN
                NULL;
        END;
        RETURN NULL;
    END;

3. Run a copy into the dummy table. No record will be inserted there, but all of them will be inserted in the original_table

psql dbname -c \copy dummy_original_table(datum,primary_key) FROM '/home/user/data.csv' delimiter E'\t'

Answer 3

Here's one solution -- import the batch file one line at a time. The performance can be much slower, but it may be sufficient for your scenario:

#!/bin/bash

input_file=./my_input.csv
tmp_file=/tmp/one-line.csv
cat $input_file | while read input_line; do
    echo "$input_line" > $tmp_file
    psql my_database \
     -c "\
     COPY my_table \
     FROM `$tmp_file` \
     DELIMITER '|'\
     CSV;\
    "
done

Additionally, you could modify the script to capture the psql stdout/stderr and exit status, and if the exit status is non-zero, echo $input_line and the captured stdout/stderr to stdin and/or append it to a file.

Answer 4

Workaround: remove the reported errant line using sed and run \copy again

Later versions of Postgres (including Postgres 13), will report the line number of the error. You can then remove that line with sed and run \copy again, e.g.,

#!/bin/bash
bad_line_number=5  # assuming line 5 is the bad line
sed ${bad_line_number}d < input.csv > filtered.csv

[per the comment from @Botond_Balázs ]