Обсуждение: Changing data types
I posted this in my last message, but have not heard anything yet so I'm wondering if it was overlooked. I need to know how to change a column from being say a varchar(9) to an integer. Does anyone know how to change the data type? Geoff
This is not for -hackers. And the answer is "no, you can't". Recreate the table with correct types and insert the old values into it. On Mon, 24 Sep 2001, Gowey, Geoffrey wrote: > I posted this in my last message, but have not heard anything yet so I'm > wondering if it was overlooked. I need to know how to change a column from > being say a varchar(9) to an integer. Does anyone know how to change the > data type? > > Geoff > > ---------------------------(end of broadcast)--------------------------- > TIP 6: Have you searched our list archives? > > http://archives.postgresql.org > >
>This is not for -hackers. How so? >And the answer is "no, you can't". Recreate the table with correct types >and insert the old values into it. You're kidding me, right? *prepares to gargle* MS Sql server can. Surely we can implement this feature or aren't we aiming to go head to head with commercial rdbms'? >>On Mon, 24 Sep 2001, Gowey, Geoffrey wrote: >> I posted this in my last message, but have not heard anything yet so I'm >> wondering if it was overlooked. I need to know how to change a column from >> being say a varchar(9) to an integer. Does anyone know how to change the >> data type?
One thought did just occur to me. It is at least theoretically possible to simplisticly migrate on column type to another by reading in the data and oid of the row into a struct, drop the column, create a new column with the correct data type, and populate. This is ugly, but it is better than saying "no, you can't". Geoff -----Original Message----- From: Gowey, Geoffrey Sent: Monday, September 24, 2001 10:53 AM To: 'Alex Pilosov'; Gowey, Geoffrey Cc: pgsql-hackers@postgresql.org Subject: RE: [HACKERS] Changing data types >This is not for -hackers. How so? >And the answer is "no, you can't". Recreate the table with correct types >and insert the old values into it. You're kidding me, right? *prepares to gargle* MS Sql server can. Surely we can implement this feature or aren't we aiming to go head to head with commercial rdbms'? >>On Mon, 24 Sep 2001, Gowey, Geoffrey wrote: >> I posted this in my last message, but have not heard anything yet so I'm >> wondering if it was overlooked. I need to know how to change a column from >> being say a varchar(9) to an integer. Does anyone know how to change the >> data type?
"Gowey, Geoffrey" wrote: > > I posted this in my last message, but have not heard anything yet so I'm > wondering if it was overlooked. I need to know how to change a column from > being say a varchar(9) to an integer. Does anyone know how to change the > data type? create temptableas select col_a, col_b, varchar9col_c::int, col_d from originaltable ; drop table originaltable; alter table temptable rename to originaltable; and then create all indexes and constraints. --------------- Hannu
"Gowey, Geoffrey" wrote: > > One thought did just occur to me. It is at least theoretically possible to > simplisticly migrate on column type to another by reading in the data and > oid of the row into a struct, drop the column, create a new column with the > correct data type, and populate. This is ugly, but it is better than saying > "no, you can't". The DROP COLUMN part is the one that is what's really hard. It is not currently supported in postgreSQL Supporting it comes up now and then, but as the solution (changing system tables and then rewriting the whole table) is considered ugly in current implementation it has always windled down to not doing it. The way to manually change column type is something like: alter table mytable add column newcolumn int; update table set newcolumn = oldcolumn; alter table rename oldcolumn to __del__001; alter table rename newcolumn to oldcolumn; but you can't DROP COLUMN without recreating the TABLE ------------ Hannu
"Gowey, Geoffrey" wrote: > > >This is not for -hackers. > > How so? > > >And the answer is "no, you can't". Recreate the table with correct types > >and insert the old values into it. > > You're kidding me, right? *prepares to gargle* MS Sql server can. Surely > we can implement this feature or aren't we aiming to go head to head with > commercial rdbms'? To be honest I am very surprised that MS SQL supports that, but then again Microsoft is so used to doing everything so utterly wrong, they have to design all their products with the ability to support fundamental design error corrections on the fly. I would be surprised if Oracle, DB2, or other "industrial grade" databases could do this. Needing to change a column from a varchar to an integer is a huge change and a major error in design. Adding a column, updating a column with a conversion routine, dropping the old column, and renaming the new column to the old column name is probably supported, but, geez, I have been dealing with SQL for almost 8 years and I have never needed to do that.
On Mon, 24 Sep 2001, mlw wrote: > To be honest I am very surprised that MS SQL supports that, but then > again Microsoft is so used to doing everything so utterly wrong, they > have to design all their products with the ability to support > fundamental design error corrections on the fly. > > I would be surprised if Oracle, DB2, or other "industrial grade" > databases could do this. Needing to change a column from a varchar to > an integer is a huge change and a major error in design. Actually they do. Its not a such a great deal, same as adding a column and dropping a column. If you can do that, you can do modification of type. The sticky thing is dropping a column. There are two options, and postgresql developers just can't make up their mind :P) a) keep old column data in database (wasted space, but fast) b) immediately 'compress' table, removing old data (slow, needs a lot of space for compression) Option a) was implemented once, but kludgy, and had a few kinks, and it was removed. Option b) plain sucks :P) -alex
On Mon, 24 Sep 2001, Rod Taylor wrote: > Out of curiosity how was option a) implemented? I could envision > supporting multiple versions of a tuple style to be found within a > table (each described in pg_attribute). Gradually these would be > upgraded through normal use. Check the archives (look for "DROP COLUMN" and "Hiroshi Inoue", author of original patch). > One would expect that keeping multiple versions of a tuple structure > inside a single table to be slower than normal for selects, but I > don't think it would require marking the rows themselves -- just base > it on the max and min transactions in the table at that time. Vacuum > would have to push the issue (5k tuples at a time?) of upgrading some > of the tuples each time it's run in order to enfore that they were all > gone before XID wrap. Background vacuum is ideal for that (if > implemented). Drop all constraints, indexes and the name (change to > $1 or something) of the column immediatly. Vacuum can determine when > XID Min in a table is > XID Max of another version and drop the > information from pg_attribute. I think it was done by setting attribute_id to negative, essentially hiding it from most code, instead of having two tuple versions, but I really am not very familiar. Check archives :) > Obviously affected: > - pg_attribute, and anything dealing with it (add XID Max, XID Min > wraps for known ranges) > - storage machanism. On read of a tuple attempt to make it fit latest > version (XID Max is NULL) by ignoring select fields. > > I'll have to leave it up to the pros as to whether it can be done, > should be done, and what else it'll affect. > > I suppose this was option a) that was removed due to it's kludgyness > :)
Unfortunately, some of the head aches I have been encountering require me to be able to do such oddities (example: my money column type not working with the pgsql odbc driver). It's not just limited to a varchar to int conversion that was just an example. There's a bunch of things that I need to be able to do (and I would gladly help with the coding if I knew where to start). Geoff -----Original Message----- From: mlw [mailto:markw@mohawksoft.com] Sent: Monday, September 24, 2001 9:25 PM To: Gowey, Geoffrey Cc: 'Alex Pilosov'; pgsql-hackers@postgresql.org Subject: Re: Changing data types "Gowey, Geoffrey" wrote: > > >This is not for -hackers. > > How so? > > >And the answer is "no, you can't". Recreate the table with correct types > >and insert the old values into it. > > You're kidding me, right? *prepares to gargle* MS Sql server can. Surely > we can implement this feature or aren't we aiming to go head to head with > commercial rdbms'? To be honest I am very surprised that MS SQL supports that, but then again Microsoft is so used to doing everything so utterly wrong, they have to design all their products with the ability to support fundamental design error corrections on the fly. I would be surprised if Oracle, DB2, or other "industrial grade" databases could do this. Needing to change a column from a varchar to an integer is a huge change and a major error in design. Adding a column, updating a column with a conversion routine, dropping the old column, and renaming the new column to the old column name is probably supported, but, geez, I have been dealing with SQL for almost 8 years and I have never needed to do that.
> The sticky thing is dropping a column. There are two options, and > postgresql developers just can't make up their mind :P) > > a) keep old column data in database (wasted space, but fast) > b) immediately 'compress' table, removing old data (slow, needs a lot of > space for compression) > > Option a) was implemented once, but kludgy, and had a few kinks, and it > was removed. Option b) plain sucks :P) Out of curiosity how was option a) implemented? I could envision supporting multiple versions of a tuple style to be found within a table (each described in pg_attribute). Gradually these would be upgraded through normal use. I'm personally not fond of the option b) due to the time involved in completing the action. Not only is space an issue, but locking the database up for a day while removing a column isn't the nicest thing to do -- rename, make nullable, drop all constraints and try to ignore it right? One would expect that keeping multiple versions of a tuple structure inside a single table to be slower than normal for selects, but I don't think it would require marking the rows themselves -- just base it on the max and min transactions in the table at that time. Vacuum would have to push the issue (5k tuples at a time?) of upgrading some of the tuples each time it's run in order to enfore that they were all gone before XID wrap. Background vacuum is ideal for that (if implemented). Drop all constraints, indexes and the name (change to $1 or something) of the column immediatly. Vacuum can determine when XID Min in a table is > XID Max of another version and drop the information from pg_attribute. Obviously affected: - pg_attribute, and anything dealing with it (add XID Max, XID Min wraps for known ranges) - storage machanism. On read of a tuple attempt to make it fit latest version (XID Max is NULL) by ignoring select fields. I'll have to leave it up to the pros as to whether it can be done, should be done, and what else it'll affect. I suppose this was option a) that was removed due to it's kludgyness :)