Comment by branko_d

For inserts, you cannot escape writing into the base table and all indexes. However, my understanding is that for updates PostgreSQL has a write amplification problem due to the fact that each time a row is updated this creates a new row (to implement MVCC), and a new physical location in the heap, so all indexes need to be updated to point to the new location, even those not containing the updated columns.

OTOH, with a heap-less (aka. clustered, aka. index organized) table, you would only have to update the indexes containing the columns that are actually being updated. You don't need to touch any other index. Furthermore, only if you are updating a key column would you physically "move" the entry into a different part of the B-tree. If you update an included column (PK columns are automatically "included" in all secondary indexes, even if not explicitly mentioned in the index definition), you can do that in-place, without moving the entry.

Here is how this works in SQL Server - consider the following example:

    CREATE TABLE T (

        ID int,
        NAME nvarchar(255) NOT NULL,
        AMOUNT int NOT NULL,

        CONSTRAINT T_PK PRIMARY KEY (ID)

    );

    GO

    CREATE INDEX T_I1 ON T (NAME);

    GO

    CREATE INDEX T_I2 ON T (AMOUNT);

Now, doing this...

    UPDATE T SET AMOUNT = 42 WHERE ID = 100;

...will only write to T_PK and T_I2, but not T_I1. Furthermore T_PK's entry will not need to be moved to a different place in the B-tree. SQL Server uses row versioning similar to PostgreSQL, so it's conceivable that PostgreSQL could behave similarly to SQL Server if it supported clustered (index-organized) tables.