我们有一个嵌套的sql代码，可从hana数据库的多个表中查询性能数据。需要帮助以了解代码的执行方式
通过以下链接了解嵌套的工作方式：https://mode.com/sql-tutorial/sql-subqueries/
SELECT
START_TIME,
BLK_PHASE_START_TIME,
CRIT_PHASE_START_TIME,
END_TIME,
HOST,
LPAD(PORT, 5) PORT,
LPAD(NUM_SAVEPOINTS, 10) SAVEPOINTS,
MAP(PURPOSE, 'DROP_SNAPSHOT', 'DS', 'NORMAL', 'N', 'SNAPSHOT_FOR_REPLICATION', 'SFR', 'SNAPSHOT_FOR_BACKUP', 'SFB', 'SNAPSHOT_FOR_RESUMERE...', 'SRR', 'SNAPSHOT_FOR_SECONDARY', 'SFS', PURPOSE) P,
MAP(INITIATION, 'EXCECUTED_EXPLICITLY', 'E', 'EXECUTED_EXPLICITLY', 'E', 'TRIGGERED_TIMEBASED', 'T', INITIATION) I,
LPAD(TO_DECIMAL(AVG_RETRIES, 10, 2), 11) AVG_RETRIES,
LPAD(TO_DECIMAL(AVG_DURATION_S, 12, 2), 11) AVG_TOTAL_S,
LPAD(TO_DECIMAL(AVG_BLOCKING_PHASE_S, 12, 2), 9) AVG_BLK_S,
LPAD(TO_DECIMAL(AVG_WAIT_FOR_LOCK_PHASE_S, 12, 2), 10) AVG_LOCK_S,
LPAD(TO_DECIMAL(AVG_CRITICAL_PHASE_S, 12, 2), 10) AVG_CRIT_S,
LPAD(TO_DECIMAL(ROUND(AVG_SIZE_MB), 11, 0), 11) AVG_SIZE_MB,
LPAD(TO_DECIMAL(MB_PER_S, 10, 2), 8) MB_PER_S,
LPAD(TO_DECIMAL(RS_SIZE_PCT, 12, 2), 11) RS_SIZE_PCT
FROM
( SELECT
CASE WHEN NUM_SAVEPOINTS = 1 THEN CRIT_PHASE_START_TIME ELSE 'various' END CRIT_PHASE_START_TIME,
CASE WHEN NUM_SAVEPOINTS = 1 THEN BLK_PHASE_START_TIME ELSE 'various' END BLK_PHASE_START_TIME,
CASE WHEN NUM_SAVEPOINTS = 1 THEN END_TIME ELSE 'various' END END_TIME,
START_TIME,
HOST,
PORT,
NUM_SAVEPOINTS,
INITIATION,
PURPOSE,
AVG_RETRIES,
AVG_DURATION_S,
AVG_BLOCKING_PHASE_S,
AVG_WAIT_FOR_LOCK_PHASE_S,
AVG_CRITICAL_PHASE_S,
AVG_SIZE_MB,
MAP(SUM_DURATION_S, 0, 0, SUM_SIZE_MB / SUM_DURATION_S) MB_PER_S,
RS_SIZE_PCT
FROM
( SELECT
CASE
WHEN BI.AGGREGATE_BY = 'NONE' OR INSTR(BI.AGGREGATE_BY, 'TIME') != 0 THEN
CASE
WHEN BI.TIME_AGGREGATE_BY LIKE 'TS%' THEN
TO_VARCHAR(ADD_SECONDS(TO_TIMESTAMP('2014/01/01 00:00:00', 'YYYY/MM/DD HH24:MI:SS'), FLOOR(SECONDS_BETWEEN(TO_TIMESTAMP('2014/01/01 00:00:00',
'YYYY/MM/DD HH24:MI:SS'), CASE BI.TIMEZONE WHEN 'UTC' THEN ADD_SECONDS(SP.START_TIME, SECONDS_BETWEEN(CURRENT_TIMESTAMP, CURRENT_UTCTIMESTAMP)) ELSE SP.START_TIME END) / SUBSTR(BI.TIME_AGGREGATE_BY, 3)) * SUBSTR(BI.TIME_AGGREGATE_BY, 3)), 'YYYY/MM/DD HH24:MI:SS')
ELSE TO_VARCHAR(CASE BI.TIMEZONE WHEN 'UTC' THEN ADD_SECONDS(SP.START_TIME, SECONDS_BETWEEN(CURRENT_TIMESTAMP, CURRENT_UTCTIMESTAMP)) ELSE SP.START_TIME END, BI.TIME_AGGREGATE_BY)
END
ELSE 'any'
END START_TIME,
CASE WHEN BI.AGGREGATE_BY = 'NONE' OR INSTR(BI.AGGREGATE_BY, 'HOST') != 0 THEN SP.HOST ELSE MAP(BI.HOST, '%', 'any', BI.HOST) END HOST,
CASE WHEN BI.AGGREGATE_BY = 'NONE' OR INSTR(BI.AGGREGATE_BY, 'PORT') != 0 THEN TO_VARCHAR(SP.PORT) ELSE MAP(BI.PORT, '%', 'any', BI.PORT) END PORT,
CASE WHEN BI.AGGREGATE_BY = 'NONE' OR INSTR(BI.AGGREGATE_BY, 'INITIATION') != 0 THEN SP.INITIATION ELSE MAP(BI.INITIATION, '%', 'any', BI.INITIATION) END INITIATION,
CASE WHEN BI.AGGREGATE_BY = 'NONE' OR INSTR(BI.AGGREGATE_BY, 'PURPOSE') != 0 THEN SP.PURPOSE ELSE MAP(BI.PURPOSE, '%', 'any', BI.PURPOSE) END PURPOSE,
TO_VARCHAR(MIN(SP.BLOCKING_PHASE_START_TIME), 'YYYY/MM/DD HH24:MI:SS') BLK_PHASE_START_TIME,
TO_VARCHAR(MIN(SP.CRITICAL_PHASE_START_TIME), 'YYYY/MM/DD HH24:MI:SS') CRIT_PHASE_START_TIME,
TO_VARCHAR(MIN(SP.END_TIME), 'YYYY/MM/DD HH24:MI:SS') END_TIME,
COUNT(*) NUM_SAVEPOINTS,
AVG(SP.DURATION) / 1000000 AVG_DURATION_S,
SUM(SP.DURATION) / 1000000 SUM_DURATION_S,
AVG(SP.BLOCKING_PHASE_DURATION) / 1000000 AVG_BLOCKING_PHASE_S,
AVG(SP.WAIT_FOR_LOCK_DURATION) / 1000000 AVG_WAIT_FOR_LOCK_PHASE_S,
AVG(SP.CRITICAL_PHASE_DURATION) / 1000000 AVG_CRITICAL_PHASE_S,
AVG(SP.TOTAL_SIZE) / 1024 / 1024 AVG_SIZE_MB,
SUM(SP.TOTAL_SIZE) / 1024 / 1024 SUM_SIZE_MB,
AVG(MAP(SP.TOTAL_SIZE, 0, 0, SP.FLUSHED_ROWSTORE_SIZE / SP.TOTAL_SIZE * 100)) RS_SIZE_PCT,
AVG(SP.RETRY_COUNT) AVG_RETRIES
FROM
( SELECT
HOST,
PORT,
TIMEZONE,
BEGIN_TIME,
END_TIME,
MIN_BLOCKING_PHASE_DURATION_S,
MIN_WAIT_FOR_LOCK_PHASE_DURATION_S,
MIN_CRITICAL_PHASE_DURATION_S,
MIN_SAVEPOINT_DURATION_S,
MIN_WRITE_SIZE_GB,
INITIATION,
PURPOSE,
DATA_SOURCE,
AGGREGATE_BY,
MAP(TIME_AGGREGATE_BY,
'NONE', 'YYYY/MM/DD HH24:MI:SS:FF7',
'HOUR', 'YYYY/MM/DD HH24',
'DAY', 'YYYY/MM/DD (DY)',
'HOUR_OF_DAY', 'HH24',
TIME_AGGREGATE_BY ) TIME_AGGREGATE_BY
FROM
( SELECT /* Modification section */
TO_TIMESTAMP('2019/06/14 00:00:00', 'YYYY/MM/DD HH24:MI:SS') BEGIN_TIME,
TO_TIMESTAMP('2019/06/14 17:00:00', 'YYYY/MM/DD HH24:MI:SS') END_TIME,
'SERVER' TIMEZONE, /* SERVER, UTC */
'%' HOST,
'%' PORT,
-1 MIN_BLOCKING_PHASE_DURATION_S,
-1 MIN_WAIT_FOR_LOCK_PHASE_DURATION_S,
-1 MIN_CRITICAL_PHASE_DURATION_S,
-1 MIN_SAVEPOINT_DURATION_S,
-1 MIN_WRITE_SIZE_GB,
'%' INITIATION,
'%' PURPOSE,
'CURRENT' DATA_SOURCE, /* CURRENT, HISTORY */
'NONE' AGGREGATE_BY, /* HOST, PORT, TIME, INITIATION, PURPOSE or comma separated combinations, NONE for no aggregation */
'TS900' TIME_AGGREGATE_BY /* HOUR, DAY, HOUR_OF_DAY or database time pattern, TS<seconds> for time slice, NONE for no aggregation */
FROM
DUMMY
)
) BI,
( SELECT
'CURRENT' DATA_SOURCE,
HOST,
PORT,
START_TIME,
ADD_SECONDS(CRITICAL_PHASE_START_TIME, - CRITICAL_PHASE_WAIT_TIME / 1000000) BLOCKING_PHASE_START_TIME,
CRITICAL_PHASE_START_TIME,
ADD_SECONDS(START_TIME, DURATION / 1000000) END_TIME,
DURATION,
CRITICAL_PHASE_WAIT_TIME WAIT_FOR_LOCK_DURATION,
CRITICAL_PHASE_DURATION,
CRITICAL_PHASE_WAIT_TIME + CRITICAL_PHASE_DURATION BLOCKING_PHASE_DURATION,
TOTAL_SIZE,
FLUSHED_ROWSTORE_SIZE,
INITIATION,
PURPOSE,
PREPARE_FLUSH_RETRY_COUNT RETRY_COUNT
FROM
M_SAVEPOINTS
UNION ALL
SELECT DISTINCT
'HISTORY' DATA_SOURCE,
HOST,
PORT,
START_TIME,
ADD_SECONDS(CRITICAL_PHASE_START_TIME, - CRITICAL_PHASE_WAIT_TIME / 1000000) BLOCKING_PHASE_START_TIME,
CRITICAL_PHASE_START_TIME,
ADD_SECONDS(START_TIME, DURATION / 1000000) END_TIME,
DURATION,
CRITICAL_PHASE_WAIT_TIME WAIT_FOR_LOCK_DURATION,
CRITICAL_PHASE_DURATION,
CRITICAL_PHASE_WAIT_TIME + CRITICAL_PHASE_DURATION BLOCKING_PHASE_DURATION,
TOTAL_SIZE,
FLUSHED_ROWSTORE_SIZE,
INITIATION,
PURPOSE,
PREPARE_FLUSH_RETRY_COUNT RETRY_COUNT
FROM
_SYS_STATISTICS.HOST_SAVEPOINTS
) SP
WHERE
SP.HOST LIKE BI.HOST AND
TO_VARCHAR(SP.PORT) LIKE BI.PORT AND
CASE BI.TIMEZONE WHEN 'UTC' THEN ADD_SECONDS(SP.START_TIME, SECONDS_BETWEEN(CURRENT_TIMESTAMP, CURRENT_UTCTIMESTAMP)) ELSE SP.START_TIME END BETWEEN BI.BEGIN_TIME AND BI.END_TIME AND
SP.INITIATION LIKE BI.INITIATION AND
SP.PURPOSE LIKE BI.PURPOSE AND
( BI.MIN_BLOCKING_PHASE_DURATION_S = -1 OR
SP.BLOCKING_PHASE_DURATION / 1000000 >= BI.MIN_BLOCKING_PHASE_DURATION_S) AND
( BI.MIN_WAIT_FOR_LOCK_PHASE_DURATION_S = -1 OR
SP.WAIT_FOR_LOCK_DURATION / 1000000 >= BI.MIN_WAIT_FOR_LOCK_PHASE_DURATION_S ) AND
( BI.MIN_CRITICAL_PHASE_DURATION_S = -1 OR
SP.CRITICAL_PHASE_DURATION / 1000000 >= BI.MIN_CRITICAL_PHASE_DURATION_S ) AND
( BI.MIN_SAVEPOINT_DURATION_S = -1 OR
SP.DURATION / 1000000 >= BI.MIN_SAVEPOINT_DURATION_S ) AND
( BI.MIN_WRITE_SIZE_GB = -1 OR
SP.TOTAL_SIZE / 1024 / 1024 / 1024 >= BI.MIN_WRITE_SIZE_GB ) AND
SP.DATA_SOURCE LIKE BI.DATA_SOURCE
GROUP BY
CASE
WHEN BI.AGGREGATE_BY = 'NONE' OR INSTR(BI.AGGREGATE_BY, 'TIME') != 0 THEN
CASE
WHEN BI.TIME_AGGREGATE_BY LIKE 'TS%' THEN
TO_VARCHAR(ADD_SECONDS(TO_TIMESTAMP('2014/01/01 00:00:00', 'YYYY/MM/DD HH24:MI:SS'), FLOOR(SECONDS_BETWEEN(TO_TIMESTAMP('2014/01/01 00:00:00',
'YYYY/MM/DD HH24:MI:SS'), CASE BI.TIMEZONE WHEN 'UTC' THEN ADD_SECONDS(SP.START_TIME, SECONDS_BETWEEN(CURRENT_TIMESTAMP, CURRENT_UTCTIMESTAMP)) ELSE SP.START_TIME END) / SUBSTR(BI.TIME_AGGREGATE_BY, 3)) * SUBSTR(BI.TIME_AGGREGATE_BY, 3)), 'YYYY/MM/DD HH24:MI:SS')
ELSE TO_VARCHAR(CASE BI.TIMEZONE WHEN 'UTC' THEN ADD_SECONDS(SP.START_TIME, SECONDS_BETWEEN(CURRENT_TIMESTAMP, CURRENT_UTCTIMESTAMP)) ELSE SP.START_TIME END, BI.TIME_AGGREGATE_BY)
END
ELSE 'any'
END,
CASE WHEN BI.AGGREGATE_BY = 'NONE' OR INSTR(BI.AGGREGATE_BY, 'HOST') != 0 THEN SP.HOST ELSE MAP(BI.HOST, '%', 'any', BI.HOST) END,
CASE WHEN BI.AGGREGATE_BY = 'NONE' OR INSTR(BI.AGGREGATE_BY, 'PORT') != 0 THEN TO_VARCHAR(SP.PORT) ELSE MAP(BI.PORT, '%', 'any', BI.PORT) END,
CASE WHEN BI.AGGREGATE_BY = 'NONE' OR INSTR(BI.AGGREGATE_BY, 'INITIATION') != 0 THEN SP.INITIATION ELSE MAP(BI.INITIATION, '%', 'any', BI.INITIATION) END,
CASE WHEN BI.AGGREGATE_BY = 'NONE' OR INSTR(BI.AGGREGATE_BY, 'PURPOSE') != 0 THEN SP.PURPOSE ELSE MAP(BI.PURPOSE, '%', 'any', BI.PURPOSE) END
)
)
ORDER BY
START_TIME DESC,
HOST,
PORT
需要帮助来了解代码块的执行顺序。