create new unique ID in postgres - php

Question

I have a table
id | entry | field | value
1 | 1 | name | Egon
2 | 1 | sname | Smith
3 | 1 | city | Los Angeles
4 | 2 | name | Stephe
5 | 2 | sname | Mueller
6 | 2 | city | New York
where id is the PK and autoincrements. Lines with the same entry belong together, they are kind of a dataset. I want to add new lines for a new entry (which should have the value 3).
My Question is: how can I obtain the next value for entry in a way, that it is unique?
At the moment im using something like
SELECT MAX(entry)+1
but when two queries are made at the same time, I'll get entry = "3" for both of them. I'm coding in PHP.

Answer 1

Use the sequence that you already got as a unique identifier at INSERT (don't leave this task to PHP):
INSERT INTO "my_table" ("entry", "field", "value") VALUES (currval(('"my_current_id_sequence"'::text)::regclass), 'any_field_info', 'any_value_info');
This way postgres will use the right number EVERYTIME.
Another aproach is to create a sequence and use it on INSERT as well, just use nextval(('"my_new_sequence"'::text)::regclass) as the value of entry and you can even use it as a default value.
If you need to know which value was used just add RETURNING entry to your INSERT
CREATE SEQUENCE alt_serial START 101;
ALTER TABLE "my_table"
ALTER COLUMN "entry" SET DEFAULT nextval(('"alt_serial"'::text)::regclass);
INSERT INTO "my_table" ("entry", "field", "value") VALUES (DEFAULT, 'any_field_info', 'any_value_info') RETURNING entry;

Answer 2

you can do some kind of loop that increments the entry every loop. If you want to add more lines with the same entry, pass more arguments to it in 1 loop.
class foo{
protected $foo = [];
public function set_foo($arguments = []){
$this->foo = $arguments;
}
public function insert_rows(){
$entry = // last entry from the db (number)- your logic here
$num_of_foos = count($this->foo);
$i = 0;
$query = 'INSERT INTO table(entry, field, value ) VALUES';
while($i<$num_of_foos){
if($i>1){$query .= ',';} // adds the comma if loop repeats more than once
$query .= '("';
$query .= $entry; // entry number for the first item
$query .= ',' . $this->foo[$i] . ',' . $some_Value;
$query .= '")';
$i++;
} // end of while loop
$entry++;
} // end of function insert_rows()
} // end of class foo
//now all you need to do is set the foo
$foo = new foo;
$foo->set_foo('lalala'); // sets one argument with new entry
$foo->set_foo('jajaja'); // sets another argument with new entry
$foo->set_foo('lala', 'jaja'); // sets two arguments with the same entry

Answer 3

You need to:
create a sequence
CREATE SEQUENCE table_entry_seq;
Then, when you need to add a row with a new entry, get the next value of the sequence:
SELECT nextval(('"table_entry_seq"'::text)::regclass)