CTE based sequence generation with HSQLDB

Question

I am using a recursive common table expression to fetch a batch of sequence number. The following query works with Postgres, SQL Server and H2 (minus the VALUES part).

WITH RECURSIVE t(n, level_num) AS (
    SELECT next value for seq_parent_id as n,
           1 as level_num
      FROM (VALUES(0))

    UNION ALL

    SELECT next value for seq_parent_id as n,
           level_num + 1 as level_num
      FROM t
     WHERE level_num < ?)
SELECT n FROM t

However with HSQLDB 2.4.0 I get the following exception

java.sql.SQLSyntaxErrorException: user lacks privilege or object not found: T
    at org.hsqldb.jdbc.JDBCUtil.sqlException(Unknown Source)
    at org.hsqldb.jdbc.JDBCUtil.sqlException(Unknown Source)
    at org.hsqldb.jdbc.JDBCStatement.fetchResult(Unknown Source)
    at org.hsqldb.jdbc.JDBCStatement.executeQuery(Unknown Source)
    ... 
Caused by: org.hsqldb.HsqlException: user lacks privilege or object not found: T
    at org.hsqldb.error.Error.error(Unknown Source)
    at org.hsqldb.error.Error.error(Unknown Source)
    at org.hsqldb.ParserDQL.readTableName(Unknown Source)
    at org.hsqldb.ParserDQL.readTableOrSubquery(Unknown Source)
    at org.hsqldb.ParserDQL.XreadTableReference(Unknown Source)
    at org.hsqldb.ParserDQL.XreadFromClause(Unknown Source)
    at org.hsqldb.ParserDQL.XreadTableExpression(Unknown Source)
    at org.hsqldb.ParserDQL.XreadQuerySpecification(Unknown Source)
    at org.hsqldb.ParserDQL.XreadSimpleTable(Unknown Source)
    at org.hsqldb.ParserDQL.XreadQueryPrimary(Unknown Source)
    at org.hsqldb.ParserDQL.XreadQueryTerm(Unknown Source)
    at org.hsqldb.ParserDQL.XreadSetOperation(Unknown Source)
    at org.hsqldb.ParserDQL.XreadQueryExpressionBody(Unknown Source)
    at org.hsqldb.ParserDQL.XreadQueryExpression(Unknown Source)
    at org.hsqldb.ParserDQL.XreadSubqueryTableBody(Unknown Source)
    at org.hsqldb.ParserDQL.XreadTableNamedSubqueryBody(Unknown Source)
    at org.hsqldb.ParserDQL.XreadQueryExpression(Unknown Source)
    at org.hsqldb.ParserDQL.compileCursorSpecification(Unknown Source)
    at org.hsqldb.ParserCommand.compilePart(Unknown Source)
    at org.hsqldb.ParserCommand.compileStatements(Unknown Source)
    at org.hsqldb.Session.executeDirectStatement(Unknown Source)
    at org.hsqldb.Session.execute(Unknown Source)
    ... 37 more

This specific use case could also be solved with a combination of UNNEST and SEQUENCE_ARRAY but I'd like to avoid having to introduce an HSQLDB specific code path.

Answer 1

I'd start with the simplest form of the recursive query without use of sequences and with hard-coded limit and then gradually add extra bits to it.

Based on the example in the docs With Clause and Recursive Queries the syntax should look like this:

WITH RECURSIVE
t(level_num)
AS
(
    VALUES(1)

    UNION ALL

    SELECT
        level_num + 1
    FROM t
    WHERE level_num < 10
)
SELECT level_num 
FROM t
;

By the way, the docs say:

HyperSQL limits recursion to 265 rounds. If this is exceeded, an error is raised.

I'd try the simplest query, like the one above, make sure that it works, then try it with, say, 1000 instead of 10 and see what error it returns. If it is the same error that you had originally, then you found the reason.

---

A side note: I'd use a permanent table of numbers instead of generating them on the fly recursively for the task of this kind. We have a table with 100K numbers in our system. It is simple and would work in any DBMS. Populate it once and use as needed. I know that in SQL Server recursive query is significantly slower (in this kind of task), I don't know about HyperSQL though. Also, the limit of recursion depth to just 265 is rather harsh. Most likely, with such low limit on the recursion depth it would be impossible to detect any difference in performance. But, again, is 265 numbers enough for your purposes?

Answer 2

HSQLDB has an issue with the 'UNION ALL'. In above referred example "With Clause and Recursive Queries" there is no 'UNION ALL', but only 'UNION' (note that documentation might be changed). In this thread there is some discussion about it. But at the moment I cannot get a working recursive statement in HSQLDB that uses UNION ALL.

So use UNION for HSQLDB v 2.3.2+