In this post we are going to discuss some 11gR2 changes to materialized view logs that are aimed at increasing the performance of the fast-refresh engine of materialized views (MVs), especially the on-commit variant.
The MV logs, in 10gr2, now comes in two flavours: the traditional (and still the default) timestamp-based one and the brand new commit SCN-based one; you choose the latter type by specifing the "WITH COMMIT SCN" clause at MV log creation time. Interestingly, the "old" timestamp-based implementation has been changed as well. Let's examine both with the help, as usual, of a test case.
Timestamp-based MV logs (the "old" type)
The test case configures an MV log as "log everything", that is, it activates all the logging options:
create materialized view log on test_t1 with sequence, rowid, primary key (x1) including new values;
In pre-11gR2 (e.g. in 188.8.131.52, 10.2.0.4), the MV log columns were:
pk1 number(22) x1 number(22) m_row$$ varchar2(255) sequence$$ number(22) snaptime$$ date(7) dmltype$$ varchar2(1) old_new$$ varchar2(1) change_vector$$ raw(255)
now in 11gR2 (10.2.0.1):
pk1 number(22) x1 number(22) m_row$$ varchar2(255) sequence$$ number(22) snaptime$$ date(7) dmltype$$ varchar2(1) old_new$$ varchar2(1) change_vector$$ raw(255) xid$$ number(22)
the only difference is the new column xid$$ (transaction id) that uniquely identifies the transaction that made the changes to the row. For the curious, the number is a combination of the elements of the triplet (undo segment number, undo slot, undo sequence); it is simply the binary concatenation of the three numbers shifted by (48, 32, 0) bits respectively (as checked in the script).
The xid$$ column is used by the 11gR2 on-commit fast refresh engine, which can now easily retrieve the changes made by the just-committed transaction by its xid; at the opposite, the on-demand fast refresh one keeps using snaptime$$ as it did in previous versions. I will speak about this in more detail in an upcoming post.
Commit SCN-based MV logs (the "new" type in 11gR2)
Let's recreate the same MV log, this time adding the commit SCN clause (new in 11GR2):
create materialized view log on test_t1 with sequence, rowid, primary key (x1), COMMIT SCN including new values;
The columns of the MV log are:
pk1 number(22) x1 number(22) m_row$$ varchar2(255) sequence$$ number(22) dmltype$$ varchar2(1) old_new$$ varchar2(1) change_vector$$ raw(255) xid$$ number(22)
so, the only difference from the 11gR2 timestamp-based case is that snaptime$$ is no longer a column of the MV log; the only difference from the pre-11gR2 is that snaptime$$ has been replaced with xid$$.
For this log flavour only, the mapping between the xid that modified the table and its commit-time SCN is now tracked in a new view, all_summap (probably named after "SUMmary MAP", "summary" being yet another synonym for "MV"), which is (as of 184.108.40.206) a straight "select *" of the dictionary table sys.snap_xcmt$. To illustrate, the script makes one insert, one update and one delete on the base table, which translates into 4 rows inside the MV log with the same xid:
SQL> select distinct xid$$ from mlog$_test_t1; XID$$ --------------------- 1126024460895690
after the commit, we get
SQL> select * from all_summap where xid in (select xid$$ from mlog$_test_t1); XID COMMIT_SCN --------------------- ---------- 1126024460895690 2885433
hence, it is now possible to know the infinite-precision time (the SCN) when every modification became visible to an external observer (the commit SCN) by simply joining the MV log and all_summap (or sys.snap_xcmt$). Note that the commit SCN is not propagated to the MV log at all.
commit SCN-based MV logs for on-demand fast refresh
This new xid$$ column and commit-SCN mapping table are leveraged by the fast refresh of on-demand MVs as follows (on-commit ones do not need the SCN as they know exactly the xid of the committed transaction; again we will see that in an upcoming post).
With "old style" timestamp-based MV logs, the refresh is performed by using a "mark-and-propagate" algorithm, which is essentially (check this post for some additional details):
1) new log rows are inserted with snaptime$$=4000 A.D;
2) at refresh time, a snapshot of the new rows is taken, that is, all new rows are marked with snaptime$$=sysdate;
3) all modifications whose snaptime$$ is between the date of the last refresh (excluded) and sysdate(included) are propagated to the MV;
4) all obsolete log rows are deleted, that is, all rows whose snaptime$$ is less than or equal the lowest of all refresh times are removed from the log.
With "new style" SCN-based MV logs, the algorithm is, instead:
1) new log rows are inserted with xid$$=transaction id of modifing transaction;
2) at refresh time, the current SCN is retrieved (no snapshot is performed);
3) all modifications whose xid maps to a row in all_summap whose commit_scn is between the SCN of the last refresh (excluded) and the retrieved current SCN(included) are propagated to the MV;
4) obsolete rows are removed from the log as before, this time using the SCN instead of snaptime$$.
The main advantage is that the snapshot is not performed, thus removing the redo and undo generated by the update, and obviously the log visit (usually a full table scan) as well - at the cost of an additional join with all_summap (or sys.snap_xcmt$) later; if the join is calculated efficiently, that is very likely advantageous "in general" (but as always, it depends on your scenario).
It might be (rarely) beneficial to index xid$$, as it is (rarely) beneficial to index snaptime$$. In that case, having no snapshot performed reduces both the undo and redo generated for the index maintenance.
As a side and "philosophical" note, it is also worth noting that the new logging mechanism records more information - now we know which transactions modified the table and the infinite-precision time (the SCN) of modifications, and this is much more informative about the history of the logged table than the mostly meaningless refresh time contained in snaptime$$. This is definitely a better utilization of storage.
I plan to blog about how the new MV log impact fast refreshes in 11gR2 in the near future, focusing on join-only MVs; so stay tuned if you're interested.