PostgreSQL Error: Relation already exists

问题:

I am trying to create a table that was dropped previously.

But when I do the CREATE TABLE A ... I am getting below error:

Relation 'A' already exists.

I verified doing SELECT * FROM A, but then I got another error:

Relation 'A' does not exists.

I already tried to find it in \dS+ listing all relations, and it is not there.
To complicate this, I have tested this by creating this table in another database and I got the same error. I am thinking that could be an error when this table was dropped. Any ideas?

Here is the code: I'm using a generated code from Power SQL. I have the same error without using the sequence. It just works when I change the name and in this case I can not do that.

CREATE SEQUENCE csd_relationship_csd_relationship_id_seq;
CREATE TABLE csd_relationship (
    csd_relationship_id INTEGER NOT NULL DEFAULT nextval('csd_relationship_csd_relationship_id_seq'::regclass),  
    type_id INTEGER NOT NULL,
    object_id INTEGER NOT NULL,
    CONSTRAINT csd_relationship PRIMARY KEY (csd_relationship_id)
);

回答1:

I finally discover the error. The problem is that the primary key constraint name is equal the table name. I don know how postgres represents constraints, but I think the error "Relation already exists" was being triggered during the creation of the primary key constraint because the table was already declared. But because of this error, the table wasnt created at the end.

回答2:

There should be no single quotes here 'A'. Single quotes are for string literals: 'some value'.
Either use double quotes to preserve the upper case spelling of "A":

CREATE TABLE "A" ...

Or don't use quotes at all:

CREATE TABLE A ...

which is identical to

CREATE TABLE a ...

because all unquoted identifiers are folded to lower case automatically in PostgreSQL.

---

You could avoid problems with the index name completely by using simpler syntax:

CREATE TABLE csd_relationship (
    csd_relationship_id serial PRIMARY KEY,
    type_id integer NOT NULL,
    object_id integer NOT NULL
);

Does the same as your original query, only it avoids naming conflicts automatically. It picks the next free identifier automatically. More about the serial type in the manual.

回答3:

You cannot create a table with a name that is identical to an existing table or view in the cluster. To modify an existing table, use ALTER TABLE (link), or to drop all data currently in the table and create an empty table with the desired schema, issue DROP TABLE before CREATE TABLE.

It could be that the sequence you are creating is the culprit. In PostgreSQL, sequences are implemented as a table with a particular set of columns. If you already have the sequence defined, you should probably skip creating it. Unfortunately, there's no equivalent in CREATE SEQUENCE to the IF NOT EXISTS construct available in CREATE TABLE. By the looks of it, you might be creating your schema unconditionally, anyways, so it's reasonable to use

DROP TABLE IF EXISTS csd_relationship;
DROP SEQUENCE IF EXISTS csd_relationship_csd_relationship_id_seq;

before the rest of your schema update; In case it isn't obvious, This will delete all of the data in the csd_relationship table, if there is any

回答4:

In my case, I had a sequence with the same name.

回答5:

In my case, it wasn't until I PAUSEd the batch file and scrolled up a bit, that wasn't the only error I had gotten. My DROP command had become DROP and so the table wasn't dropping in the first place (thus the relation did indeed still exist). The  I've learned is called a Byte Order Mark (BOM). Opening this in Notepad++, re-save the SQL file with Encoding set to UTM-8 without BOM and it runs fine.

回答6:

Another reason why you might get errors like "relation already exists" is if the DROP command did not execute correctly.

One reason this can happen is if there are other sessions connected to the database which you need to close first.

回答7:

In my case I was migrating from 9.5 to 9.6. So to restore a database, I was doing :

sudo -u postgres psql -d databse -f dump.sql

Of course it was executing on the old postgreSQL database where there are datas! If your new instance is on port 5433, the correct way is :

sudo -u postgres psql -d databse -f dump.sql -p 5433