Postgresql – Update column with value of another column or another column

postgresqlupdate

With PostgreSQL (I'm using the version 9.1) is it possible to do mass update with a single query a column value with the value of another column, but if the other column value is null, to use the value of a third column, and if the third one is absent to use the current datetime (all the column have type timestamp)

I need to change

columnA columnB columnC
null    foo     bar
null    null    baz
null    null    null

columnA columnB columnC
foo     foo     bar
baz     null    baz
quz     null    null

where quz is the current datetime.

Best Answer

You can use COALESCE() function to do the update:

UPDATE ...
SET columnA = COALESCE(columnB, columnC, now());

COALESCE will return the first non-null value in the list that you provide.

Related Solutions

Postgresql – How to add FK on a column of composite type in PostgreSQL

This avoids the syntax error:

create type bbb as (col2 int);

create table aaa (
  col1 serial primary key
);

create table ccc (
  id serial primary key,
  col3 bbb references aaa --will reference the primary key of aaa
);

Even without the syntax error, you still get:

"Schema Creation Failed: ERROR: foreign key constraint "ccc_col3_fkey" cannot be implemented
  Detail: Key columns "col3" and "col1" are of incompatible types: bbb and integer.: "

This works though:

create type bbb as (col2 int);

create table aaa (
  col1 bbb primary key
);

create table ccc (
  id serial primary key,
  col3 bbb references aaa
);

But now you don't have a sequence. Also, you can't create sequence as...

So you are going to have to live without sequences on those tables or just use an int instead of a bbb.

Postgresql – Most efficient way to do many UPDATES in Postgresql

If you don't want to interfere with other activity, the one UPDATE at a time in autocommit mode is very likely the best option. You should probably set synchronous_commit=off in that session (and only that session).

The indexes are going to slow you down, perhaps by a lot depending on your RAM and your IO system. But if the index is necessary for the other actions you don't want to interfere with, then there isn't anything you can do about it.

But since the fid is not yet correctly populated, the index on it is probably not actually useful to the concurrent processes you want avoid interfering with, as they haven't been changed yet to rely on that column being accurate. If that is the case, you can drop that index to gain speed, and build it in bulk later. The same probably applies to the foreign key constraint.

Once that index is gone, your updates can proceed via HOT (Heap Only Tuples) updates provided each block has enough free space. In that case, the updates will not have to do maintenance on the primary key index, either, saving that much more IO. To maximize the likelihood that this will work optimally, it is important that each UPDATE be its own transaction. That way one UPDATE can reuse space freed up by an earlier one.

Also, your WHERE clause should probably be like:

WHERE id=345 and fid is not null;

That way if the script gets interrupted, you can re-run it with minimal damage.

Since you seem to be running this on a test system already, then an EXPLAIN (ANALYZE,BUFFERS) of some of the updates would be helpful, especially with track_io_timing set to on.

Best Answer

Related Solutions

Postgresql – How to add FK on a column of composite type in PostgreSQL

Postgresql – Most efficient way to do many UPDATES in Postgresql

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