One of the long standing limitations of partitions is that you can't have foreign keys pointing to them.
Let's see if I can make it possible to have some kind of constraint that would do the same thing as fkey.
Tag: partitions
Waiting for PostgreSQL 12 – Allow multi-inserts during COPY into a partitioned table
On 1st of August 2018, Peter Eisentraut committed patch:
Allow multi-inserts during COPY into a partitioned table CopyFrom allows multi-inserts to be used for non-partitioned tables, but this was disabled for partitioned tables. The reason for this appeared to be that the tuple may not belong to the same partition as the previous tuple did. Not allowing multi-inserts here greatly slowed down imports into partitioned tables. These could take twice as long as a copy to an equivalent non-partitioned table. It seems wise to do something about this, so this change allows the multi-inserts by flushing the so-far inserted tuples to the partition when the next tuple does not belong to the same partition, or when the buffer fills. This improves performance when the next tuple in the stream commonly belongs to the same partition as the previous tuple. In cases where the target partition changes on every tuple, using multi-inserts slightly slows the performance. To get around this we track the average size of the batches that have been inserted and adaptively enable or disable multi-inserts based on the size of the batch. Some testing was done and the regression only seems to exist when the average size of the insert batch is close to 1, so let's just enable multi-inserts when the average size is at least 1.3. More performance testing might reveal a better number for, this, but since the slowdown was only 1-2% it does not seem critical enough to spend too much time calculating it. In any case it may depend on other factors rather than just the size of the batch. Allowing multi-inserts for partitions required a bit of work around the per-tuple memory contexts as we must flush the tuples when the next tuple does not belong the same partition. In which case there is no good time to reset the per-tuple context, as we've already built the new tuple by this time. In order to work around this we maintain two per-tuple contexts and just switch between them every time the partition changes and reset the old one. This does mean that the first of each batch of tuples is not allocated in the same memory context as the others, but that does not matter since we only reset the context once the previous batch has been inserted. Author: David Rowley <david.rowley@2ndquadrant.com>
Waiting for PostgreSQL 11 – Improve run-time partition pruning to handle any stable expression.
On 10th of June 2018, Tom Lane committed patch:
Improve run-time partition pruning to handle any stable expression. The initial coding of the run-time-pruning feature only coped with cases where the partition key(s) are compared to Params. That is a bit silly; we can allow it to work with any non-Var-containing stable expression, as long as we take special care with expressions containing PARAM_EXEC Params. The code is hardly any longer this way, and it's considerably clearer (IMO at least). Per gripe from Pavel Stehule. David Rowley, whacked around a bit by me Discussion: https://postgr.es/m/CAFj8pRBjrufA3ocDm8o4LPGNye9Y+pm1b9kCwode4X04CULG3g@mail.gmail.com
Waiting for PostgreSQL 11 – Support partition pruning at execution time
On 7th of April 2018, Alvaro Herrera committed patch:
Support partition pruning at execution time Existing partition pruning is only able to work at plan time, for query quals that appear in the parsed query. This is good but limiting, as there can be parameters that appear later that can be usefully used to further prune partitions. This commit adds support for pruning subnodes of Append which cannot possibly contain any matching tuples, during execution, by evaluating Params to determine the minimum set of subnodes that can possibly match. We support more than just simple Params in WHERE clauses. Support additionally includes: 1. Parameterized Nested Loop Joins: The parameter from the outer side of the join can be used to determine the minimum set of inner side partitions to scan. 2. Initplans: Once an initplan has been executed we can then determine which partitions match the value from the initplan. Partition pruning is performed in two ways. When Params external to the plan are found to match the partition key we attempt to prune away unneeded Append subplans during the initialization of the executor. This allows us to bypass the initialization of non-matching subplans meaning they won't appear in the EXPLAIN or EXPLAIN ANALYZE output. For parameters whose value is only known during the actual execution then the pruning of these subplans must wait. Subplans which are eliminated during this stage of pruning are still visible in the EXPLAIN output. In order to determine if pruning has actually taken place, the EXPLAIN ANALYZE must be viewed. If a certain Append subplan was never executed due to the elimination of the partition then the execution timing area will state "(never executed)". Whereas, if, for example in the case of parameterized nested loops, the number of loops stated in the EXPLAIN ANALYZE output for certain subplans may appear lower than others due to the subplan having been scanned fewer times. This is due to the list of matching subnodes having to be evaluated whenever a parameter which was found to match the partition key changes. This commit required some additional infrastructure that permits the building of a data structure which is able to perform the translation of the matching partition IDs, as returned by get_matching_partitions, into the list index of a subpaths list, as exist in node types such as Append, MergeAppend and ModifyTable. This allows us to translate a list of clauses into a Bitmapset of all the subpath indexes which must be included to satisfy the clause list. Author: David Rowley, based on an earlier effort by Beena Emerson Reviewers: Amit Langote, Robert Haas, Amul Sul, Rajkumar Raghuwanshi, Jesper Pedersen Discussion: https://postgr.es/m/CAOG9ApE16ac-_VVZVvv0gePSgkg_BwYEV1NBqZFqDR2bBE0X0A@mail.gmail.com
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Waiting for PostgreSQL 11 – Allow UPDATE to move rows between partitions.
On 19th of January 2018, Robert Haas committed patch:
Allow UPDATE to move rows between partitions. When an UPDATE causes a row to no longer match the partition constraint, try to move it to a different partition where it does match the partition constraint. In essence, the UPDATE is split into a DELETE from the old partition and an INSERT into the new one. This can lead to surprising behavior in concurrency scenarios because EvalPlanQual rechecks won't work as they normally did; the known problems are documented. (There is a pending patch to improve the situation further, but it needs more review.) Amit Khandekar, reviewed and tested by Amit Langote, David Rowley, Rajkumar Raghuwanshi, Dilip Kumar, Amul Sul, Thomas Munro, Álvaro Herrera, Amit Kapila, and me. A few final revisions by me. Discussion: http://postgr.es/m/CAJ3gD9do9o2ccQ7j7+tSgiE1REY65XRiMb=yJO3u3QhyP8EEPQ@mail.gmail.com
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Waiting for PostgreSQL 10 – Implement table partitioning.
I had two month delay related to some work, but now I can finally write about:
On 7th of December, Robert Haas committed patch:
Implement table partitioning. Table partitioning is like table inheritance and reuses much of the existing infrastructure, but there are some important differences. The parent is called a partitioned table and is always empty; it may not have indexes or non-inherited constraints, since those make no sense for a relation with no data of its own. The children are called partitions and contain all of the actual data. Each partition has an implicit partitioning constraint. Multiple inheritance is not allowed, and partitioning and inheritance can't be mixed. Partitions can't have extra columns and may not allow nulls unless the parent does. Tuples inserted into the parent are automatically routed to the correct partition, so tuple-routing ON INSERT triggers are not needed. Tuple routing isn't yet supported for partitions which are foreign tables, and it doesn't handle updates that cross partition boundaries. Currently, tables can be range-partitioned or list-partitioned. List partitioning is limited to a single column, but range partitioning can involve multiple columns. A partitioning "column" can be an expression. Because table partitioning is less general than table inheritance, it is hoped that it will be easier to reason about properties of partitions, and therefore that this will serve as a better foundation for a variety of possible optimizations, including query planner optimizations. The tuple routing based which this patch does based on the implicit partitioning constraints is an example of this, but it seems likely that many other useful optimizations are also possible. Amit Langote, reviewed and tested by Robert Haas, Ashutosh Bapat, Amit Kapila, Rajkumar Raghuwanshi, Corey Huinker, Jaime Casanova, Rushabh Lathia, Erik Rijkers, among others. Minor revisions by me.
Continue reading Waiting for PostgreSQL 10 – Implement table partitioning.
Partitioning – what? why? how?
Recently I noticed that more and more cases that I deal with could use some partitioning. And while theoretically most people know about it, it's definitely not a very well-understood feature, and sometimes people are scared of it.
So, I'll try to explain, to my best knowledge, what it is, why one would want to use it, and how to actually make it happen.
Waiting for 9.5 – Allow foreign tables to participate in inheritance. – A.K.A. PostgreSQL got sharding.
On 22nd of March, Tom Lane committed patch:
Allow foreign tables to participate in inheritance. Foreign tables can now be inheritance children, or parents. Much of the system was already ready for this, but we had to fix a few things of course, mostly in the area of planner and executor handling of row locks. As side effects of this, allow foreign tables to have NOT VALID CHECK constraints (and hence to accept ALTER ... VALIDATE CONSTRAINT), and to accept ALTER SET STORAGE and ALTER SET WITH/WITHOUT OIDS. Continuing to disallow these things would've required bizarre and inconsistent special cases in inheritance behavior. Since foreign tables don't enforce CHECK constraints anyway, a NOT VALID one is a complete no-op, but that doesn't mean we shouldn't allow it. And it's possible that some FDWs might have use for SET STORAGE or SET WITH OIDS, though doubtless they will be no-ops for most. An additional change in support of this is that when a ModifyTable node has multiple target tables, they will all now be explicitly identified in EXPLAIN output, for example: Update on pt1 (cost=0.00..321.05 rows=3541 width=46) Update on pt1 Foreign Update on ft1 Foreign Update on ft2 Update on child3 -> Seq Scan on pt1 (cost=0.00..0.00 rows=1 width=46) -> Foreign Scan on ft1 (cost=100.00..148.03 rows=1170 width=46) -> Foreign Scan on ft2 (cost=100.00..148.03 rows=1170 width=46) -> Seq Scan on child3 (cost=0.00..25.00 rows=1200 width=46) This was done mainly to provide an unambiguous place to attach "Remote SQL" fields, but it is useful for inherited updates even when no foreign tables are involved. Shigeru Hanada and Etsuro Fujita, reviewed by Ashutosh Bapat and Kyotaro Horiguchi, some additional hacking by me
Waiting for 8.4 – window functions
A long overdue post about new functionality. At this moment it is no longer such new, as it was committed on 28th of December (yes, I know, I should have written it earlier, Sorry).
On this day Tom Lane committed patch by Hitoshi Harada which adds support for so called window functions:
Support window functions a la SQL:2008. Hitoshi Harada, with some kibitzing from Heikki and Tom.