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Re: SEARCH and CYCLE clauses - Mailing list pgsql-hackers

From Pavel Stehule
Subject Re: SEARCH and CYCLE clauses
Date
Msg-id CAFj8pRD8hn1vEdX+EMXQ2hz2pkfyJ2a9cV6R8mbaLYn-MRexdQ@mail.gmail.com
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In response to SEARCH and CYCLE clauses  (Peter Eisentraut <peter.eisentraut@2ndquadrant.com>)
Responses Re: SEARCH and CYCLE clauses
Re: SEARCH and CYCLE clauses
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Hi

út 22. 9. 2020 v 20:01 odesílatel Peter Eisentraut <peter.eisentraut@2ndquadrant.com> napsal:
I have implemented the SEARCH and CYCLE clauses.

This is standard SQL syntax attached to a recursive CTE to compute a
depth- or breadth-first ordering and cycle detection, respectively.
This is just convenience syntax for what you can already do manually.
The original discussion about recursive CTEs briefly mentioned these as
something to do later but then it was never mentioned again.

SQL specifies these in terms of syntactic transformations, and so that's
how I have implemented them also, mainly in the rewriter.

I have successfully tested this against examples I found online that
were aimed at DB2.

The contained documentation and the code comment in rewriteHandler.c
explain the details.

I am playing with this patch. It looks well, but I found some issues (example is from attached data.sql)

WITH recursive destinations (departure, arrival, connections, cost) AS
    (SELECT f.departure, f.arrival, 0, price
            FROM flights f
            WHERE f.departure = 'New York'
     UNION ALL
     SELECT r.departure, b.arrival, r.connections + 1,
                r.cost + b.price
            FROM destinations r, flights b
            WHERE r.arrival = b.departure) cycle departure, arrival set is_cycle to true default false using path
   
SELECT *
  FROM destinations ;
;

The result is correct. When I tried to use UNION instead UNION ALL, the pg crash

Program received signal SIGABRT, Aborted.
0x00007f761338ebc5 in raise () from /lib64/libc.so.6
(gdb) bt
#0  0x00007f761338ebc5 in raise () from /lib64/libc.so.6
#1  0x00007f76133778a4 in abort () from /lib64/libc.so.6
#2  0x000000000090e7eb in ExceptionalCondition (conditionName=<optimized out>, errorType=<optimized out>, fileName=<optimized out>,
    lineNumber=<optimized out>) at assert.c:67
#3  0x00000000007205e7 in generate_setop_grouplist (targetlist=targetlist@entry=0x7f75fce5d018, op=<optimized out>, op=<optimized out>)
    at prepunion.c:1412
#4  0x00000000007219d0 in generate_recursion_path (pTargetList=0x7fff073ee728, refnames_tlist=<optimized out>, root=0xf90bd8, setOp=0xf90840)
    at prepunion.c:502
#5  plan_set_operations (root=0xf90bd8) at prepunion.c:156
#6  0x000000000070f79b in grouping_planner (root=0xf90bd8, inheritance_update=false, tuple_fraction=<optimized out>) at planner.c:1886
#7  0x0000000000712ea7 in subquery_planner (glob=<optimized out>, parse=<optimized out>, parent_root=<optimized out>, hasRecursion=<optimized out>,
    tuple_fraction=0) at planner.c:1015
#8  0x000000000071a614 in SS_process_ctes (root=0xf7abd8) at subselect.c:952
#9  0x00000000007125d4 in subquery_planner (glob=glob@entry=0xf8a010, parse=parse@entry=0xf6cf20, parent_root=parent_root@entry=0x0,
    hasRecursion=hasRecursion@entry=false, tuple_fraction=tuple_fraction@entry=0) at planner.c:645
#10 0x000000000071425b in standard_planner (parse=0xf6cf20, query_string=<optimized out>, cursorOptions=256, boundParams=<optimized out>)
    at planner.c:405
#11 0x00000000007e5f68 in pg_plan_query (querytree=0xf6cf20,
    query_string=query_string@entry=0xea6370 "WITH recursive destinations (departure, arrival, connections, cost) AS \n    (SELECT f.departure, f.arrival, 0, price\n", ' ' <repeats 12 times>, "FROM flights f \n", ' ' <repeats 12 times>, "WHERE f.departure = 'New York' \n     UNION "...,
    cursorOptions=cursorOptions@entry=256, boundParams=boundParams@entry=0x0) at postgres.c:875
#12 0x00000000007e6061 in pg_plan_queries (querytrees=0xf8b690,
    query_string=query_string@entry=0xea6370 "WITH recursive destinations (departure, arrival, connections, cost) AS \n    (SELECT f.departure, f.arrival, 0, price\n", ' ' <repeats 12 times>, "FROM flights f \n", ' ' <repeats 12 times>, "WHERE f.departure = 'New York' \n     UNION "...,
    cursorOptions=cursorOptions@entry=256, boundParams=boundParams@entry=0x0) at postgres.c:966
#13 0x00000000007e63b8 in exec_simple_query (
    query_string=0xea6370 "WITH recursive destinations (departure, arrival, connections, cost) AS \n    (SELECT f.departure, f.arrival, 0, price\n", ' ' <repeats 12 times>, "FROM flights f \n", ' ' <repeats 12 times>, "WHERE f.departure = 'New York' \n     UNION "...) at postgres.c:1158
#14 0x00000000007e81e4 in PostgresMain (argc=<optimized out>, argv=<optimized out>, dbname=<optimized out>, username=<optimized out>) at postgres.c:4309
#15 0x00000000007592b9 in BackendRun (port=0xecaf20) at postmaster.c:4541
#16 BackendStartup (port=0xecaf20) at postmaster.c:4225
#17 ServerLoop () at postmaster.c:1742
#18 0x000000000075a0ed in PostmasterMain (argc=<optimized out>, argv=0xea0c90) at postmaster.c:1415
#19 0x00000000004832ec in main (argc=3, argv=0xea0c90) at main.c:209



looks so clause USING in cycle detection is unsupported for DB2 and Oracle - the examples from these databases doesn't work on PG without modifications

Regards

Pavel



 

--
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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