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\nWith Oracle you can learn something every day. Today, preparing a migration to 12.2, I found all tables had locked statistics. I learned that it is the expected behavior since 10.2 when importing metadata_only including statistics, to avoid that the automatic job gathering comes and replaces the stats by ‘0 rows’.
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\nIt is documented in DataPump Import Without Data Locks Table Statistics (Doc ID 415081.1) but as I was really surprised about that (and also frustrated to learn a 10.2 thing when migrating to 12.2) that I wanted to test myself.
I create a table DEMO with statistics:
\n\nSQL> connect sys/oracle@//localhost/pdb1 as sysdba\nConnected.\nSQL> create table demo.demo as select * from dual;\nTable created.\n \nSQL> create index demo.demo on demo.demo(dummy);\nIndex created.\n \nSQL> exec dbms_stats.gather_table_stats('DEMO','DEMO');\nPL/SQL procedure successfully completed.\n \nSQL> create or replace directory TMP as '/tmp';\nDirectory created.\n \nSQL> select count(*) from DEMO.DEMO;\n \n COUNT(*)\n----------\n 1\n \nSQL> select object_type from dba_objects where owner='DEMO' and object_name='DEMO';\n \nOBJECT_TYPE\n-----------------------\nTABLE\nINDEX\n \nSQL> select owner,table_name,last_analyzed,stattype_locked,num_rows from dba_tab_statistics where owner='DEMO' and table_name='DEMO';\n \nOWNER TABLE_NAME LAST_ANALYZED STATTYPE_LOCKED NUM_ROWS\n---------- -------------------- -------------------- -------------------- ----------\nDEMO DEMO 06-SEP-17 1\nI export it:
\n\nConnected to: Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production\nStarting \"SYS\".\"SYS_EXPORT_TABLE_01\": \"sys/********@//localhost/pdb1 AS SYSDBA\" tables=DEMO.DEMO directory=TMP\nProcessing object type TABLE_EXPORT/TABLE/TABLE_DATA\nProcessing object type TABLE_EXPORT/TABLE/INDEX/STATISTICS/INDEX_STATISTICS\nProcessing object type TABLE_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS\nProcessing object type TABLE_EXPORT/TABLE/STATISTICS/MARKER\nProcessing object type TABLE_EXPORT/TABLE/TABLE\nProcessing object type TABLE_EXPORT/TABLE/INDEX/INDEX\n. . exported \"DEMO\".\"DEMO\" 5.054 KB 1 rows\nMaster table \"SYS\".\"SYS_EXPORT_TABLE_01\" successfully loaded/unloaded\n******************************************************************************\nDump file set for SYS.SYS_EXPORT_TABLE_01 is:\n /tmp/expdat.dmp\nJob \"SYS\".\"SYS_EXPORT_TABLE_01\" successfully completed at Wed Sep 6 19:14:44 2017 elapsed 0 00:00:09\nAnd drop it:
\n\nSQL> connect sys/oracle@//localhost/pdb1 as sysdba\nConnected.\nSQL> drop table demo.demo;\nTable dropped.\nNow import metadata only (for example because I want to change NLS semantics before importing the data)
\n\nImport: Release 12.2.0.1.0 - Production on Wed Sep 6 19:21:28 2017\n \nCopyright (c) 1982, 2017, Oracle and/or its affiliates. All rights reserved.\n \nConnected to: Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production\nMaster table \"SYS\".\"SYS_IMPORT_TABLE_01\" successfully loaded/unloaded\nStarting \"SYS\".\"SYS_IMPORT_TABLE_01\": \"sys/********@//localhost/pdb1 AS SYSDBA\" tables=DEMO.DEMO directory=TMP content=metadata_only exclude=index\nProcessing object type TABLE_EXPORT/TABLE/TABLE\nProcessing object type TABLE_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS\nProcessing object type TABLE_EXPORT/TABLE/STATISTICS/MARKER\nJob \"SYS\".\"SYS_IMPORT_TABLE_01\" successfully completed at Wed Sep 6 19:21:39 2017 elapsed 0 00:00:11\nIf I check the statistics:
\n\nSQL> select owner,table_name,last_analyzed,stattype_locked,num_rows from dba_tab_statistics where owner='DEMO' and table_name='DEMO';\n \nOWNER TABLE_NAME LAST_ANALYZED STATTYPE_LOCKED NUM_ROWS\n---------- -------------------- -------------------- -------------------- ----------\nDEMO DEMO 06-SEP-17 ALL 1\nStats are locked. I suppose that the idea is that you have the tables with same statistics as production for example, and you can load them with a subset of data but expect the same execution plans as in production. But this is not what I want for a migration.
\nOne possibility is to unlock the stats once you have imported the data.
\nThe other possibility is to import metadata without the statistics:
\n\nConnected to: Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production\nMaster table \"SYS\".\"SYS_IMPORT_TABLE_01\" successfully loaded/unloaded\nStarting \"SYS\".\"SYS_IMPORT_TABLE_01\": \"sys/********@//localhost/pdb1 AS SYSDBA\" tables=DEMO.DEMO directory=TMP content=metadata_only exclude=index exclude=table_statistics\nProcessing object type TABLE_EXPORT/TABLE/TABLE\nJob \"SYS\".\"SYS_IMPORT_TABLE_01\" successfully completed at Wed Sep 6 21:11:03 2017 elapsed 0 00:00:03\nThen the table statistics are not locked:
\n\nOWNER TABLE_NAME LAST_ANALYZED STATTYPE_LOCKED NUM_ROWS\n---------- -------------------- -------------------- -------------------- ----------\nDEMO DEMO\nOnce you have changed what you want on the tables, you import the data (table_exists_action=truncate) and then you import the remaining: indexes, ref_constraints, triggers.
\nThis is where you can also add include=table_statistics:
\nStarting \"SYS\".\"SYS_IMPORT_TABLE_01\": \"sys/********@//localhost/pdb1 AS SYSDBA\" tables=DEMO.DEMO directory=TMP table_exists_action=truncate include=index include=table_statistics\nProcessing object type TABLE_EXPORT/TABLE/INDEX/INDEX\nProcessing object type TABLE_EXPORT/TABLE/INDEX/STATISTICS/INDEX_STATISTICS\nProcessing object type TABLE_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS\nProcessing object type TABLE_EXPORT/TABLE/STATISTICS/MARKER\nSo that you have the statistics from the source, unlocked.
\n\nOWNER TABLE_NAME LAST_ANALYZED STATTYPE_LOCKED NUM_ROWS\n---------- -------------------- -------------------- -------------------- ----------\nDEMO DEMO 06-SEP-17 1\n