Um einen Ausführungsplan der DB2-Datenbank anzuzeigen, sind drei Schritte nötig. Der erste ist ein einmaliges Setup.
Benötigte Tabellen anlegen (Einmalig)
Der empfohlene Weg, die notwendigen System-Tabellen anzulegen, ist, diese Prozedur aufzurufen:
CALL SYSPROC.SYSINSTALLOBJECTS('EXPLAIN', 'C',
CAST (NULL AS VARCHAR(128)), CAST (NULL AS VARCHAR(128)))Dadurch werden die benötigten Tabellen im
SYSTOOLS-Schema angelegt (z.B.
SYSTOOLS.EXPLAIN_STREAM).
In der IBM-Dokumentation werden auch andere Möglichkeiten gezeigt, diese Tabellen zu installieren.
Einen Ausführungsplan erstellen und speichern
Durch voranstellen von explain plan
for kann man den Ausführungsplan für eine SQL-Anweisung in
den oben erwähnten Tabellen abspeichern.
Der Ausführungsplan wird dabei aber noch nicht angezeigt, sondern bloß in der Datenbank abgespeichert.
EXPLAIN PLAN FOR SELECT 1 FROM sysibm.sysdummy1Einen gespeicherten Ausführungsplan anzeigen
IBM stellt einige Werkzeuge zur Verfügung, um einen abgespeicherten Ausführungsplan anzuzeigen. Das Anzeigeformat dieser Werkzeuge weicht jedoch deutlich von dem anderer Datenbanken ab, sodass wir hier einen View vorstellen, der genau auf unsere Bedürfnisse zugeschnitten ist, und den Ausführungsplan ähnlich zu anderen Datenbanken darstellt. Die DB2-Dokumentation legt es durchaus nahe, eigene Anzeige-Methoden zu verwenden.
Der View last_explained (Definition siehe unten)
liefert den Ausführungsplan für die letzte SQL-Anweisung, die mittels
explain plan for vom aktuellen
Benutzer analysiert wurde. Dabei ist zu beachten, dass dabei nicht auf
die aktuelle Session eingeschränkt wird.
Mit diesem View kann der Ausführungsplan wie folgt dargestellt werden:
SELECT * FROM last_explainedDas Ergebnis könnte so aussehen. Die Bedeutung der einzelnen Operationen wird auf der nächsten Seite erklärt.
Explain Plan
----------------------------------------------
ID | Operation | Rows | Cost
1 | RETURN | | 0
2 | TBSCAN GENROW | 1 of 1 (100.00%) | 0
Predicate Information
Explain plan by Markus Winand - NO WARRANTY
http://use-the-index-luke.com/s/last_explained Warnung
Dieser View ist experimentell. Wir übernehmen keinerlei Gewähr!
Der View verwendet die Explain-Tabellen im SYSTOOLS-Schema. Um andere Explain-Tabellen zu verwenden, muss man den View manuell anpassen.
-- Copyright (c) 2014-2017, Markus Winand - NO WARRANTY
-- Modifications by Ember Crooks - NO WARRANTY
-- Info & license: http://use-the-index-luke.com/s/last_explained
--
--#SET TERMINATOR ;
CREATE OR REPLACE VIEW last_explained AS
WITH tree(operator_ID, level, path, explain_time, cycle)
AS
(
SELECT 1 operator_id
, 0 level
, CAST('001' AS VARCHAR(1000)) path
, max(explain_time) explain_time
, 0
FROM SYSTOOLS.EXPLAIN_OPERATOR O
WHERE O.EXPLAIN_REQUESTER = SESSION_USER
UNION ALL
SELECT s.source_id
, level + 1
, tree.path || '/' || LPAD(CAST(s.source_id AS VARCHAR(3)), 3, '0') path
, tree.explain_time
, POSITION('/' || LPAD(CAST(s.source_id AS VARCHAR(3)), 3, '0') || '/' IN path USING OCTETS)
FROM tree
, SYSTOOLS.EXPLAIN_STREAM S
WHERE s.target_id = tree.operator_id
AND s.explain_time = tree.explain_time
AND S.Object_Name IS NULL
AND S.explain_requester = SESSION_USER
AND tree.cycle = 0
AND level < 100
)
SELECT *
FROM (
SELECT "Explain Plan"
FROM (
SELECT CAST( LPAD(id, MAX(LENGTH(id)) OVER(), ' ')
|| ' | '
|| RPAD(operation, MAX(LENGTH(operation)) OVER(), ' ')
|| ' | '
|| LPAD(rows, MAX(LENGTH(rows)) OVER(), ' ')
|| ' | '
-- Don't show ActualRows columns if there are no actuals available at all
|| CASE WHEN COUNT(ActualRows) OVER () > 1 -- the heading 'ActualRows' is always present, so "1" means no OTHER values
THEN LPAD(ActualRows, MAX(LENGTH(ActualRows)) OVER(), ' ') || ' | '
ELSE ''
END
|| LPAD(cost, MAX(LENGTH(cost)) OVER(), ' ')
AS VARCHAR(100)) "Explain Plan"
, path
FROM (
SELECT 'ID' ID
, 'Operation' Operation
, 'Rows' Rows
, 'ActualRows' ActualRows
, 'Cost' Cost
, '0' Path
FROM SYSIBM.SYSDUMMY1
-- TODO: UNION ALL yields duplicate. where do they come from?
UNION
SELECT CAST(tree.operator_id as VARCHAR(254)) ID
, CAST(LPAD(' ', tree.level, ' ')
|| CASE WHEN tree.cycle = 1
THEN '(cycle) '
ELSE ''
END
|| COALESCE (
TRIM(O.Operator_Type)
|| COALESCE(' (' || argument || ')', '')
|| ' '
|| COALESCE(S.Object_Name,'')
, ''
)
AS VARCHAR(254)) AS OPERATION
, COALESCE(CAST(rows AS VARCHAR(254)), '') Rows
, CAST(ActualRows as VARCHAR(254)) ActualRows -- note: no coalesce
, COALESCE(CAST(CAST(O.Total_Cost AS BIGINT) AS VARCHAR(254)), '') Cost
, path
FROM tree
LEFT JOIN ( SELECT i.source_id
, i.target_id
, CAST(CAST(ROUND(o.stream_count) AS BIGINT) AS VARCHAR(12))
|| ' of '
|| CAST (total_rows AS VARCHAR(12))
|| CASE WHEN total_rows > 0
AND ROUND(o.stream_count) <= total_rows THEN
' ('
|| LPAD(CAST (ROUND(ROUND(o.stream_count)/total_rows*100,2)
AS NUMERIC(5,2)), 6, ' ')
|| '%)'
ELSE ''
END rows
, CASE WHEN act.actual_value is not null then
CAST(CAST(ROUND(act.actual_value) AS BIGINT) AS VARCHAR(12))
|| ' of '
|| CAST (total_rows AS VARCHAR(12))
|| CASE WHEN total_rows > 0 THEN
' ('
|| LPAD(CAST (ROUND(ROUND(act.actual_value)/total_rows*100,2)
AS NUMERIC(5,2)), 6, ' ')
|| '%)'
ELSE NULL
END END ActualRows
, i.object_name
, i.explain_time
FROM (SELECT MAX(source_id) source_id
, target_id
, MIN(CAST(ROUND(stream_count,0) AS BIGINT)) total_rows
, CAST(LISTAGG(object_name) AS VARCHAR(50)) object_name
, explain_time
FROM SYSTOOLS.EXPLAIN_STREAM
WHERE explain_time = (SELECT MAX(explain_time)
FROM SYSTOOLS.EXPLAIN_OPERATOR
WHERE EXPLAIN_REQUESTER = SESSION_USER
)
GROUP BY target_id, explain_time
) I
LEFT JOIN SYSTOOLS.EXPLAIN_STREAM O
ON ( I.target_id=o.source_id
AND I.explain_time = o.explain_time
AND O.EXPLAIN_REQUESTER = SESSION_USER
)
LEFT JOIN SYSTOOLS.EXPLAIN_ACTUALS act
ON ( act.operator_id = i.target_id
AND act.explain_time = i.explain_time
AND act.explain_requester = SESSION_USER
AND act.ACTUAL_TYPE like 'CARDINALITY%'
)
) s
ON ( s.target_id = tree.operator_id
AND s.explain_time = tree.explain_time
)
LEFT JOIN SYSTOOLS.EXPLAIN_OPERATOR O
ON ( o.operator_id = tree.operator_id
AND o.explain_time = tree.explain_time
AND o.explain_requester = SESSION_USER
)
LEFT JOIN (SELECT LISTAGG (CASE argument_type
WHEN 'UNIQUE' THEN
CASE WHEN argument_value = 'TRUE'
THEN 'UNIQUE'
ELSE NULL
END
WHEN 'TRUNCSRT' THEN
CASE WHEN argument_value = 'TRUE'
THEN 'TOP-N'
ELSE NULL
END
WHEN 'SCANDIR' THEN
CASE WHEN argument_value != 'FORWARD'
THEN argument_value
ELSE NULL
END
ELSE argument_value
END
, ' ') argument
, operator_id
, explain_time
FROM SYSTOOLS.EXPLAIN_ARGUMENT EA
WHERE argument_type IN ('AGGMODE' -- GRPBY
, 'UNIQUE', 'TRUNCSRT' -- SORT
, 'SCANDIR' -- IXSCAN, TBSCAN
, 'OUTERJN' -- JOINs
)
AND explain_requester = SESSION_USER
GROUP BY explain_time, operator_id
) A
ON ( a.operator_id = tree.operator_id
AND a.explain_time = tree.explain_time
)
) O
UNION ALL
VALUES ('Explain plan (c) 2014-2017 by Markus Winand - NO WARRANTY - V20171102','Z0')
, ('Modifications by Ember Crooks - NO WARRANTY','Z1')
, ('http://use-the-index-luke.com/s/last_explained','Z2')
, ('', 'A')
, ('', 'Y')
, ('Predicate Information', 'AA')
UNION ALL
SELECT CAST (LPAD(CASE WHEN operator_id = LAG (operator_id)
OVER (PARTITION BY operator_id
ORDER BY pred_order
)
THEN ''
ELSE operator_id || ' - '
END
, MAX(LENGTH(operator_id )+4) OVER()
, ' ')
|| how_applied
|| ' '
|| predicate_text
AS VARCHAR(100)) "Predicate Information"
, 'P' || LPAD(id_order, 5, '0') || pred_order path
FROM (SELECT CAST(operator_id AS VARCHAR(254)) operator_id
, LPAD(trim(how_applied)
, MAX (LENGTH(TRIM(how_applied)))
OVER (PARTITION BY operator_id)
, ' '
) how_applied
-- next: capped to length 80 to avoid
-- SQL0445W Value "..." has been truncated. SQLSTATE=01004
-- error when long literal values may appear (space padded!)
, CAST(substr(predicate_text, 1, 80) AS VARCHAR(80)) predicate_text
, CASE how_applied WHEN 'START' THEN '1'
WHEN 'STOP' THEN '2'
WHEN 'SARG' THEN '3'
ELSE '9'
END pred_order
, operator_id id_order
FROM systools.explain_predicate p
WHERE explain_time = (SELECT MAX(explain_time)
FROM systools.explain_operator)
)
)
ORDER BY path
);
Seit SQL-92 hat sich einiges getan!