CPS-1574: Remove 32K limit from DB operations

Owned by Daniel Hanrahan
Last updated: Apr 20, 2023
5 min read

Background

Repository queries taking collection parameters are limited to 32K collection items.
For example, given a FragmentRepository method:

List<FragmentEntity> findAllByAnchorIdAndXpathIn(int anchorId, Collection<String> xpaths);

This would throw an exception if supplied with a collection of 32,767 xpaths.

The reason is that JPA/Hibernate queries would translate the command to the following SQL:

SELECT * FROM fragment WHERE anchor_id = ? AND xpath IN (?,?,?,?,?,?...,?)

Each collection item (xpath) is supplied as an SQL query parameter. Hibernate with Postgres has a limit of 32,767 query parameters per SQL statement, so an exception will be thrown with too many.

CPS-1574 is to remove those limits (and write tests to verify).

Tests

As part of CPS-1574, tests have been added to confirm the presence of the limit:

def 'Delete datanodes from multiple anchors limit exceeded: 32,766 (~ 2^15) anchors.'() { given: 'more than 32,766 anchor names' def anchorNames = (0..32_766).collect { "size-of-this-name-does-not-matter-for-limit-" + it } when: 'single operation is executed to delete all datanodes in given anchors' objectUnderTest.deleteDataNodes(CPS_PERFORMANCE_TEST_DATASPACE, anchorNames, OffsetDateTime.now()) then: 'a database exception is thrown' thrown(DataAccessResourceFailureException.class) }

Once the solution is implemented, these tests will be updated to show that no exception is thrown, with even more items e.g. 40,000.

Proposal #1: Batching at 32,000

This solution adds a wrapper method that partitions the input collection into lists of 32,000 and calls the original repository method on each batch:

This solution is largely self-documenting, as the 32K limit is clearly seen in the code.

This solution still involves having a separate query parameter for each collection item - we are avoiding the limit by simply splitting into multiple SQL queries:

Observations

Performance testing is inconclusive (so far)

Testing with a batch size of 1000 (not 32000) showed a performance drop of 30% at 20000 items. More testing is needed to see if this applies with batch size of 32000, or if the test involved bad luck.

Advantage: Solution is easy to understand

This code is largely self-documenting, as the 32K limit is clearly seen in the code.

Batching on repository methods versus API methods

This solution should work equally well if batching at the public API methods instead of each repository method.

Proposal #2: Postgres arrays

This solution uses a third-party library to add Postgres array support to queries.

Adding the dependency to cps-ri/pom.xml:

This solution involves creating two versions of each repository method taking a collection parameter - one taking a Java array, and one using a Collection:

This would result in a single SQL query, given any number of xpaths:

As seen, only two SQL query parameters will be ever used. This is because the array is binary-encoded and sent as a single parameter.

Testing so far has shown no noticeable performance difference with master branch.

Observations

Performance is unchanged

Testing shows there was no increase or decrease in performance compared to master branch after arrays were used.

Disadvantage: extra dependency

Extra dependencies could add possible risks of dependency issues, security issues, etc. in future.

The new dependency is an updated version of an existing dependency (hibernate-types-52) - the developer renamed the library at some point. More info here: https://vladmihalcea.com/hibernate-types-hypersistence-utils/

Disadvantage: Limited array type support

Postgres array support is limited to primitive data types and String, that is int[], long[], float[], double[], boolean[], String[]

For example, given a FragmentRepository method such as:

Changing to an array of AnchorEntity will not work. The following will throw an exception:

Rather, the method would have to be changed to use anchor IDs:

Side note: in the above case, if using auto-generated SQL from the method name, the SQL generated using the anchor ID will be simpler than when using AnchorEntity objects, as Hibernate will generate a JOIN between FragmentEntity and AnchorEntity in the first case, but will only use FragmentEntity in the last case. So while the types are more restrictive, it may result in more optimal SQL code.

Advantage: Code already using Postgres arrays can be simplified

The current FragmentRepository has code already using Postgres arrays to support "LIKE ANY" logic (using LIKE with a collection). However, it currently uses query parameters, and is thus limited to 32K parameters.

Using native Postgres array support, this code can be written directly in the FragmentRepository interface using Query annotation, without the 32K limitation:

Comparison

  • More testing is needed to confirm if list partitioning drops performance with large datasets.

  • Solution #1 is explicit using 32K batches, so easy to understand what it does

  • Solution #2 has shorter code, but not very obvious to the reader what it does

    • We could add a comment linking to this ticket to explain the 32K limit and why arrays are used

  • Solution #2 simplifies code in the cases when we need Postgres array support.

  • It's possible to use both solutions, where most appropriate.