On 04/06/2018 12:09 PM, Thomas Poty wrote:
Thank you Laurenz !

We will certainly have to change our release management.

Is there a way to identify the list of statements that have to rewrite the table.



"Adding a column with a DEFAULT clause or changing the type of an existing column will require the entire table and its indexes to be rewritten. As an exception when changing the type of an existing column, if the USING clause does not change the column contents and the old type is either binary coercible to the new type or an unconstrained domain over the new type, a table rewrite is not needed; but any indexes on the affected columns must still be rebuilt. Adding or removing a system oid column also requires rewriting the entire table. Table and/or index rebuilds may take a significant amount of time for a large table; and will temporarily require as much as double the disk space."

For the more general case of modifying a table and the locks it takes, search the above link for lock to see what locks are taken instead of the default of ACCESS EXCLUSIVE.

For what the locks mean see:


If I am right, at least these statements need to do this :
- create a unique index
- add a column with a default value



2018-04-06 17:11 GMT+02:00 Laurenz Albe <laurenz.a...@cybertec.at <mailto:laurenz.a...@cybertec.at>>:

    On Fri, 2018-04-06 at 16:58 +0200, Thomas Poty wrote:
    > Here is a bit of context : we are migrating from MySQL to PostgreSQL and 
we have about 1000 tables.
    > Some tables are quite small but some others are very large. The service 
provided to our clients
    > relies on a high avaiability with a minimum down time due to any legal 
    > So, lets imagine :
    > in Transaction 1 : I am querying Table A (select)
    > in Transaction 2 : I am trying to alter Table A ( due to our product 
    > in Transaction 3 : I am want to query Table1 (select)
    > in MySQL : Transaction 1 retrieve data in Table A.
    > Transaction 2 : is trying to alter Table A but it is blocked by 
Transaction 1
    > Transaction 3 : Transaction 1 retrieves data in Table A ( Retreiving data 
is possible until Transaction 2 commit)
    > In PostgreSQL, it is a bit different : Transaction 1 retrieve data in 
Table A.
    > Transaction 2 : is trying to alter Table A but it is blocked by 
Transaction 1
    > Transaction 3 : Transaction 3 cannot retrieve data because  Transaction 2 
did not terminate its transaction.
    > So, with MySQL, the application is able to keep working with the table 
until the alter table completed.
    > With PostgreSQL, the application will probably be blocked (until having 
the lock on this table).
    > If I understand, if the alter table takes a long time (several hours) to 
execute, clients will be blocked during several hours.
    > How do you deal with this problem? Maybe I missed something ?

    The solution is to avoid ALTER TABLE statements that have to rewrite
    the table outside of maintenance windows.

    If your transactions are short, as they should be, it should not be
    a big deal to add or drop a column, for example.

    Laurenz Albe
    Cybertec | https://www.cybertec-postgresql.com

Adrian Klaver

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