How we can eliminate duplicate insertion after postgres partitions.
As Partition key on unique constraint causing non key attribute duplicate.
EX: ID date
1 1-01-2022
1 02-01-2022
To make ID unique we have before insert trigger is the only option, Any other ways?
You cannot have a unique constraint on a partitioned table that does not contain the partitioning key. A trigger won't be a reliable solution either, because it would be subject to race conditions (concurrent inserts) unless you are running with the SERIALIZABLE isolation level.
The best you can have is unique constraints on each individual partitions.
The best you can do is fill the values from a sequence, so that the values are automatically unique, for example with an identity column.
Related
Is it possible to achieve Upsert in Postgres without using the On Conflict clause?
I have a requirement where I converted a normal table into a partitioned table with a partition key that was not part of the Primary Key when the table was non-partitioned.
Since the partition key is added to the primary key column list now, my Upsert statements are failing as the On Conflict clause is missing the partition key. But as per the requirement, I cannot add the partition key to the On Conflict clause as I will have more than one row for the previous primary key column combination in the partitioned table.
Hence, I want Upsert to be achieved without the On Conflict clause. Can someone suggest what alternatives would work.
postgres 14
I have some table:
CREATE TABLE sometable (
id integer NOT NULL PRIMARY KEY UNIQUE ,
a integer NOT NULL DEFAULT 1,
b varchar(32) UNIQUE)
PARTITION BY RANGE (id);
But when i try to execute it, i get
ERROR: unique constraint on partitioned table must include all partitioning columns
If i execute same table definition without PARTITION BY RANGE (id) and check indexes, i get:
tablename indexname indexdef
sometable, sometable_b_key, CREATE UNIQUE INDEX sometable_b_key ON public.sometable USING btree (b)
sometable, sometable_pkey, CREATE UNIQUE INDEX sometable_pkey ON public.sometable USING btree (id)
So... unique constraints exist
whats the problem? how can i fix it?
On partitioned tables, all primary keys, unique constraints and unique indexes must contain the partition expression. That is because indexes on partitioned tables are implemented by individual indexes on each partition, and there is no way to enforce uniqueness across different indexes.
If you want to use partitioning, you have to sacrifice some consistency guarantees. There is no way around that. What you can do is create unique constraints on the partitions. That will guarantee uniqueness within each partition, but not global uniqueness.
This limitation is also mentioned in the docs
5.11.2.3. Limitations The following limitations apply to partitioned tables:
Unique constraints (and hence primary keys) on partitioned tables must
include all the partition key columns. This limitation exists because
the individual indexes making up the constraint can only directly
enforce uniqueness within their own partitions; therefore, the
partition structure itself must guarantee that there are not
duplicates in different partitions.
There is no way to create an exclusion constraint spanning the whole
partitioned table. It is only possible to put such a constraint on
each leaf partition individually. Again, this limitation stems from
not being able to enforce cross-partition restrictions.
https://www.postgresql.org/docs/current/ddl-partitioning.html#DDL-PARTITIONING-DECLARATIVE
Firstly, I have a table in database USERS with almost 30 Million records in it. I have different indices for each column. But some of the column have only 2 to 3 non null values while others are Null but still their index size is 847 MB a little less than the one index that contain unique value for each row.
Can anyone know why is it like this?
Secondly, in PostgreSQL we have a index for primary key index for each column by default what if we delete that index what will be the consequences?
What that index is really use for?
As i'm searching based on values in other columns only will it be safe to delete index for primary key?
NULL values are stored in indexes just like all other values, so the first part is not surprising.
You cannot delete the primary key index, what you could do is drop the primary key constraint. But then you cannot be certain that no duplicate rows get added to the table. If you think that is no problem, look at the many questions asking for help with exactly that problem.
Every table should have a primary key.
But it might be a good idea to get rid of some other indexes if you don't need them.
There is nothing called primary key index, seems to be you are talking about unique index.
First of all you need to understand the difference between primary key and index. You can have only one primary key in a table. Primary key would be your unique identifier of each column and does not allow nulls. Index is used to speed up your fetching process on particular column and you can have one null if it is unique index. Deleting unique index in your table will not impact any thing apart from performance. Its your way of design to have index or not
The improvements to declarative range-based partitioning in version 11 seem like they can really work for my use case, but I'm not completely sure how foreign keys work with partitions.
I have tables Files -< Segments -< Entries in which there are hundreds of Segments per File and hundreds of Entries per segment, so Entries is on the order of 10,000 times the size of Files. Files have a CreationDate field and customers will define a retention period so they'll drop old Entries. All of this clearly points to date-based partitions so it's quicker to query the most recent entries first, and easy to drop old ones.
The first issue I encounter is that when I try to create the Files table, it sounds like I have to include createdDate as part of the primary key in order to have it in the RANGE partition:
CREATE TABLE Files2
(
FileId BIGSERIAL PRIMARY KEY,
createdDate timestamp with time zone,
filepath character varying COLLATE pg_catalog."default" NOT NULL
) PARTITION BY RANGE (createdDate)
WITH (
OIDS = FALSE
)
TABLESPACE pg_default;
ERROR: insufficient columns in PRIMARY KEY constraint definition
DETAIL: PRIMARY KEY constraint on table "files2" lacks column "createddate" which is part of the partition key.
If I drop the "PRIMARY KEY" from the definition of FileId, I don't get the error, but does that affect the efficiency of child lookups?
I also don't know how to declare the partitions for the child table. PARTITION BY RANGE (Files.createdDate) doesn't work.
Since it's only in version 11 that this use case would even be possible, I haven't found much information about it and would appreciate any pointers! Thanks!
I believe it is a new feature of the version 11. There is a document from which you can get the following information:
"A primary key constraint must include a partition key column. Attempting to create a primary key constraint that does not contain partitioned columns results in an error"
https://h50146.www5.hpe.com/products/software/oe/linux/mainstream/support/lcc/pdf/PostgreSQL11NewFeaturesGAen20181022-1.pdf
On Postgres, a unique index is automatically created for primary key columns. From the docs,
When an index is declared unique, multiple table rows with equal
indexed values are not allowed. Null values are not considered equal.
A multicolumn unique index will only reject cases where all indexed
columns are equal in multiple rows.
From my understanding, it seems like this index only checks uniqueness and isn't actually present for faster access when querying by primary key id's. Does this mean that this index structure doesn't consist of a sorted table (or a tree) for the primary key column? Is this correct?
In theory a unique or primary key constraint could be enforced without the presence of an index, but it would be a painful process. The index is mainly there for performance purposes.
However some databases (eg Oracle) allow a unique or primary key constraint to be supported by a non-unique index. Primarily this allows the enforcement of the constraint to be deferred until the end of a transaction, so lack of uniqueness can be permitted temporarily during a transaction, but also allows indexes to be built in parallel and with the constraint then defined as a secondary step.
Also, I'm not sure how the internals work on a PostgreSQL btree index, but all Oracle btree's are internally declared to be unique either:
on the key column(s), for an index that is intended to be UNIQUE, or
on the key column(s) plus the indexed row's ROWID, for a non-unique index.
Quite the contrary, The index is created in order to allow faster access - mainly to check for duplicates when a new record is inserted but can also be used by other queries against PK columns. The best structure for uk indexes is a btree because during the insert the index is created - If the rdbms detects collision in the leaf he will raise a unique constraint violation.