In Oracle, if we try to create two indexes on same column of same table then it gives ORA-01408: such column list already indexed.
create index tmp_idx1 on student(roll_no);
create index tmp_idx2 on student(roll_no);
ORA-01408: such column list already indexed.
But, same if we try to create in Postgres, it allows.
How can we stop this in postgres and make behavior like in Oracle?
Related
I am testing some queries on Postgresql extension Timescaledb.
The table is called timestampdb and i run some queries on that seems like this
select id13 from timestampdb where timestamp1 >='2010-01-01 00:05:00' and timestamp1<='2011-01-01 00:05:00',
select avg(id13)::numeric(10,2) from timestasmpdb where timestamp1>='2015-01-01 00:05:00' and timestamp1<='2015-01-01 10:30:00'
When i create a hypertable i do this.
create hyper_table('timestampdb','timestamp1')
The thing is that now i want to create an index on id13.
should i try something like this?:
create hyper_table('timestampdb','timestamp1') ,import data of the table and then create index on timestampdb(id13)
or something like this:
create table timestampdb,then create hypertable('timestampdb',timestamp1') ,import the data and then CREATE INDEX ON timestampdb (timestamp1,id13)
What is the correct way to do this?
You can create an index without time dimension column, since you don't require it to be unique. Including time dimension column into an index is needed if an index contains UNIQUE or is PRIMARY KEY, since TimescaleDB partitions a hypertable into chunks on the time dimension column, which is timestamp1 in the question. If partitioning key will include space dimension columns in addition to time, they will need to be included too.
So in your case the following should be sufficient after the migration to hypertable:
create index on timestampdb(id13);
The question contains two queries and none of them need index on id13. It will be valuable to create the index on id13 if you expect different queries than in the question, which will contain condition or join on id13 column.
I have a table MAIN_SCHEMA.TEST in which I created a Index on a column CHECK_ID.
CHECK_ID is also a FOREIGN_KEY constraint in TEST table.
This table contains only 50 records.
By Mistake the index got created in Default schema DEFAULT_SCHEMA.CHECK_ID_IDX.
CREATE INDEX DEFAULT_SCHEMA.CHECK_ID_IDX(CHECK_ID ASC);
So I am trying to drop this index but the drop query gets stuck for long time.
DROP INDEX DEFAULT_SCHEMA.CHECK_ID_IDX.
there are no locks on this table when I checked.
Instead of dropping and recreating the index with the right schema, could you just try to RENAME the index? It requires the existing SCHEMA.NAME pair together with the new as input. It will not move any data, but just update the metadata.
I have to write a migration command to remove a column from the index. Currently let us say I have table1 that has index on col1 and col2
I want to remove col1 from the index. I am looking at https://www.postgresql.org/docs/9.4/static/sql-alterindex.html but it does not seem I can actually just remove a column?
If yes, will it be better to remove the column and how VS
Create new Index
Drop the old index
Also, I want to do the reverse if I need to do downgrade. So just wondering how to achieve this
The ability to alter an index doesn't exist because in order to do so you would have to destroy and recreate the index with the new columns. By default, Postgres uses B-Trees to create indices and removing a column causes that B-Tree to become invalid. As a result the B-Tree needs to be built from scratch.
If you want some more details on how indices work under the hood, this is a good article: Postgres Indices Under the Hood
You’re right, you’ll have to create a new index with a single column and then drop an old index with two columns.
My database has severals table with some column type 'money'. I would like to alter all these columns (in different tables) in a single statement rather than change type column by column, to avoid omissions.
You'll have to repeat the altering query for every column.
You might want to create a program code to do that for you. You know, with loops.
In order for the database to alter all the tables atomically you should enclose all the altering queries in a transaction (PostgreSQL supports transactional DDL).
How to create indexes on MQT(materialized query table) in Db2? I haven't found this information in documentation? Is index creation syntax the same as for common tables?
After you create your MQT you have to refresh the table before you can create indexes. However, at this point it's exactly the same as creating indexes on a normal table.
There are some limitations on what type of indexes you can create on an MQT. For example, it cannot be a unique index.