How long does the ?access plan? stick around?

I have the following in a CL program:

DltF DANWORK/$DSPFD@MPg
MonMsg CPF2105 /* file not found */
RunSQL ('Create table DANWORK/$DSPFD@MPg as ( +
SELECT B.SYSTEM_TABLE_SCHEMA as SrcLibrary +
, B.SYSTEM_TABLE_NAME as SrcFile +
, B.SYSTEM_TABLE_MEMBER as SrcMbrName +
, B.LAST_SOURCE_UPDATE_TIMESTAMP as SrcUpdTS +
, B.CREATE_TIMESTAMP as SrcCrtTS +
, B.LAST_CHANGE_TIMESTAMP as SrcTTchgTS +
, B.SOURCE_TYPE as SrcMbrType +
, cast( B.PARTITION_TEXT as char(50) CCSID 37 ) as SrcMbrText +
FROM DANWORK/$INCLDSRCF A join qsys2.SysPartitionStat B +
on ( A.SRCPF_LIBR = B.SYSTEM_TABLE_SCHEMA +
and A.SRCPF_NAME = B.SYSTEM_TABLE_NAME ) +
WHERE B.CREATE_TIMESTAMP IS NOT NULL +
) WITH DATA') +
COMMIT(*NONE)

When I submit two or more jobs that call this program to the job queue, the first job takes 20 - 25 minutes. Subsequent iterations of the same job, if executed shortly after the previous job, take 30 - 35 SECONDS. After an undetermined amount of time (12 hours, in my testing, although it could be shorter), the run time for the next job returns to 20 - 25 minutes.

I can only presume an access plan is built and retained for a certain amount of time. Do I have that right? What determines how long the access plan is retained? Is it documented? I found the section "Packages and access plans" in the SQL Reference and it seems to indicate that one cannot simply create a package on demand: "Packages are produced during distributed program preparation." What the heck is that?

BTW, Index Advisor recommended building two indexes on qsys2.SysPartitionStat, but I don't have authority and I think I've heard that IBM recommends NOT to build indexes or view on IBM system files.

- Dan

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