If I have a table with columns: a, b, c and later I do a ALTER TABLE command to add a new column "d", is it possible to add it between a and b for example, and not at the end?
I heard that the position of the columns affects performance.
It's not possible to add a column between two existing columns with an ALTER TABLE statement in SQLite. This works as designed.
The new column is always appended to the end of the list of existing
columns.
As far as I know, MySQL is the only SQL (ish) dbms that lets you determine the placement of new columns.
To add a column at a specific position within a table row, use FIRST
or AFTER col_name. The default is to add the column last. You can also
use FIRST and AFTER in CHANGE or MODIFY operations to reorder columns
within a table.
But this isn't a feature I'd use regularly, so "as far as I know" isn't really very far.
With every sql platform I've seen the only way to do this is to drop the table and re-create it.
However, I question if the position of the column affects performance... In what way would it, what operations are you doing that you think it will make a difference?
I will also note that dropping the table and recreating it is often not a heavy lift. Making a backup of a table and restoring that table is easy on all major platforms so scripting a backup - drop - create - restore is an easy task for a competent DBA.
In fact I've done so often when users ask -- but I always find it a little silly. The most often reason given is the tool of choice behaves nicer when the columns are created in a certain order. (This was also #Jarad's reason below) So this is a good lesson for tool makers, make your tool able to reorder columns (and remember it between runs) -- then everyone is happy.
I use the DB.compileStatement:
sql = DB.compileStatement("INSERT INTO tableX VALUES (?,?,?);
sql.bindString(1,"value for column 1");
sql.bindString(2,"value for column 2");
sql.bindString(3,"value for column 3");
sql.executeUpdateDelete();
So there will be a big difference if order of the columns is not correct.
Unfortunately adding columns at a specific position is not possible using ALTER TABLE, at least not in SQLite. (MySQL it is possible). Workaroud is recreating the table.. (and backup and restore data)
Related
I came across a scenario where there is a column need to be added in a table. What is the Industries best practices to add a column to the existing table in Production System.
By default at the end
At appropriate position
Before the Audit fields of the table
Our data modeler has added the column and chose the default options. Is there any performance hit if the added column is used frequently.
What is the efforts to develop the script that always add the column before the audit fields as a standard?
Any help will appreciated.
It is not possible in Oracle to decide position of the new column. (well, unless you drop and recreate new table).
Note, that order of columns is not related to performance issues.
I have a table with lots of columns, and I'd like to add two more (date and time) to the front of the existing table.
There is no data in the table right now, but I'm wondering what the best way is get the table in the format I need it.
I could just drop the table and create a new one with the correct configuration, but I'm wondering if there is a better way?
This is currently not possible. You have to drop and recreate the table.
Theoretically you could add the column, drop and re-add all other columns, but that's hardly practical.
It's an ongoing discussion and an open TODO-item of the Postgres project to allow reordering of columns. But a lot of dependencies and related considerations make that hard.
Quoting the Postgres project's ToDo List:
Allow column display reordering by recording a display, storage, and
permanent id for every column?
Contrary to what some believe, the order of columns in a table is not irrelevant, for multiple reasons.
The default order is used for statements like INSERT without column definition lists.
Or SELECT *, which returns columns in the predefined order.
The composite type of the table uses the same order of columns.
The order of columns is relevant for storage optimization (padding and alignment matter). More:
Calculating and saving space in PostgreSQL
People may be confusing this with the order of rows, which in undefined in a table.
In relational databases the order of columns in a table is irrelevant
Create a view that shows you the columns in the order you want
If you still want to, drop the table and recreate it
I'm converting data from one schema to another. Each table in the source schema has a 'status' column (default NULL). When a record has been converted, I update the status column to 1. Afterwards, I can report on the # of records that are (not) converted.
While the conversion routines are still under development, I'd like to be able to quickly reset all values for status to NULL again.
An UPDATE statement on the tables is too slow (there are too many records). Does anyone know a fast alternative way to accomplish this?
The fastest way to reset a column would be to SET UNUSED the column, then add a column with the same name and datatype.
This will be the fastest way since both operations will not touch the actual table (only dictionary update).
As in Nivas' answer the actual ordering of the columns will be changed (the reset column will be the last column). If your code rely on the ordering of the columns (it should not!) you can create a view that will have the column in the right order (rename table, create view with the same name as old table, revoke grants from base table, add grants to view).
The SET UNUSED method will not reclaim the space used by the column (whereas dropping the column will free space in each block).
If the column is nullable (since default is NULL, I think this is the case), drop and add the column again?
While the conversion routines are still under development, I'd like to be able to quickly reset all values for status to NULL again.
If you are in development why do you need 70 million records? Why not develop against a subset of the data?
Have you tried using flashback table?
For example:
select current_scn from v$database;
-- 5607722
-- do a bunch of work
flashback table TABLE_NAME to scn 5607722;
What this does is ensure that the table you are working on is IDENTICAL each time you run your tests. Of course, you need to ensure you have sufficient UNDO to hold your changes.
hm. maybe add an index to the status column.
or alterately, add a new table with the primary key only in it. then insert to that table when the record is converted, and TRUNC that table to reset...
I like some of the other answers, but I just read in a tuning book that for several reasons it's often quicker to recreate the table than to do massive updates on the table. In this case, it seems ideal, since you would be writing the CREATE TABLE X AS SELECT with hopefully very few columns.
I would like to know if there's a way to add a column to an SQL Server table after it's created and in a specific position??
Thanks.
You can do that in Management-Studio. You can examine the way this is accomplished by generating the SQL-script BEFORE saving the change. Basically it's achieved by:
removing all foreign keys
creating a new table with the added column
copying all data from the old into the new table
dropping the old table
renaming the new table to the old name
recreating all the foreign keys
In addition to all the other responses, remember that you can reorder and rename columns in VIEWs. So, if you find it necessary to store the data in one format but present it in another, you can simply add the column on to the end of the table and create a single table view that reorders and renames the columns you want to show. In almost every circumstance, this view will behave exactly like the original table.
The safest way to do this is.
Create your new table with the correct column order
Copy the data from the old table.
Drop the Old Table.
The only safe way of doing that is creating a new table (with the column where you want it), migrating the data, dropping the original table, and renaming the new table to the original name.
This is what Management Studio does for you when you insert columns.
As others have pointed out you can do this by creating a temp table moving the data and droping the orginal table and then renaming the other table. This is stupid thing to do though. If your table is large, it could be very time-consuming to do this and users will be locked out during the process. This issomething you NEVER want to do to any table in production.
There is absolutely no reason to ever care what order the columns are in a table since you should not be relying on column order anyway (what if someone else did this same stupid thing?). No queries should use select * or ordinal positions to get columns. If you are doing this now, this is broken code and needs to be fixed immediately as the results are not always going to be as expected. For instance if you do insert a column where you want it and someone else is using select * for a report, suddenly the partnumber is showing up in the spot that used to hold the Price.
By doing what you want to do, you may break much more than you fix by putting the column where you personally want it. Column order in tables should always be irrelevant. You should not be doing this every time you want columns to appear in a differnt order.
With Sql Server Management Studio you can open the table in design and drag and drop the column wherever you want
As Kane says, it's not possible in a direct way. You can see how Management Studio does it by adding a column in the design mode and checking out the change script.
If the column is not in the last position, the script basically drops the table and recreates it, with the new column in the desired position.
In databases table columns don't have order.
Write proper select statement and create a view
No.
Basically, SSMS behind the scenes will copy the table, constraints, etc, drop the old table and rename the new.
The reason is simple - columns are not meant to be ordered (nor are rows), so you're always meant to list which columns you want in a result set (select * is a bit of a hack)
I wanted to know, what should i consider while deciding if i should create a new table or modify an existing table for a sql db. i use both mysql and sqlite.
-Edit- I always thought if i can put a column into a table where it makes sense and can be used by every row then i would always modify it. However at work if its a different 'release' we put it in a different table.
You can modify existing tables, as long as
you are keeping the database Normalized
you are not breaking code that uses the table
You can create new tables even if 1. and 2. are true for the following reasons:
Performance reasons
Clarity in your schema logic.
Not sure if I'm understanding your question correctly, but one thing I always try to consider is the impact on existing data.
Taking the case of an application which relies on a database...
When you update the application (including database schema updates), it is important to ensure that any existing, in-use databases will be either backwards compatible with the application, or there is way to migrate and update the existing database.
Generally if the data is in a one-to-one relationship with the existing data in the table and if the table row size is not too large already and if there aren't too many records in the table, then I usually alter the table to accept the new column.
However, suppose I want to add a column with a default value to a table where it doesn't exist. Adding it to the table with 50 million records might not be so speedy a process and it might lock up the table on production when we move the change up. In this case, putting it into a separate table and adding the records to it may work out better. In general, I wouldn't do this unless my testing has shown that adding and populating the column will take an unacceptably long time. I would prefer to keep the record together where possible.
Same thing with the overall record size. SQL server has a byte limit to the number of bytes that can be in a record, it will allow you to create a structure that is potentially larger than that, but it will not alow you to put more than the byte limit into a specific record. Further, less wide tables tend to be faster to access due to how they are stored. Frequently, people will create a table that has a one-to-one relationship (we call them extended tables in our structure) for additional columns that are not as frequnetly used. If the fields from both tables will be frequently used, often they still create two tables but have a view that will pickout all the columns needed.
And of course if the data is in a one to many relationship, you need a related table not just a new column.
Incidentally, you should always do an alter table through a script and the SSMS GUI as it is more efficient and easier to move to prod.