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Infrastructure

Truncate – 2

Following on from my earlier comments about how a truncate works in Oracle, the second oldest question about truncate (and other DDL) appeared on the OTN database forum“Why isn’t a commit required for DDL?”

Sometimes the answer to “Why” is simply “that’s just the way it is” – and that’s what it is in this case, I think.  There may have been some historic reason why Oracle Corp. implemented DDL the way they did (commit any existing transaction the session is running, then auto-commit when complete), but once the code has been around for a few years – and accumulated lots of variations – it can be very difficult to change a historic decision, no matter how silly it may now seem.

Truncate

The old question about truncate and undo (“does a truncate generate undo or not”) appeared on the OTN database forum over the week-end, and then devolved into “what really happens on a truncate”, and then carried on.

The quick answer to the traditional question is essentially this: the actual truncate activity typically generates very little undo (and redo) compared to a full delete of all the data because all it does is tidy up any space management blocks and update the data dictionary; the undo and redo generated is only about the metadata, not about the data itself.

Oops

I made a mistake a few days ago following up a question on the OTN database forum. The question was about a problem creating a hash/list composite partitioned table, and one of the respondants suggested that perhaps the problem appeared because hash/list wasn’t a legal combination.

Spot on: so I confirmed that observation and supplied a link to the official Oracle white paper that listed the combinations that were legal in 11.2 for composite partitioning.  In fact, although I was fairly sure that hash/list wasn’t legal, I had even run up a quick test to check that the attempt would fail before I’d searched online for the document.

PK Index

Here’s one of those little details that I might have known once, or maybe it wasn’t true in earlier versions of oracle, or maybe I just never noticed it and it’s “always” been true; and it’s a detail I’ll probably have forgotten again a couple of years from now.  Consider the following two ways of creating a table with primary key:

Migrated rows

I received an email recently describing a problem with a query which was running a full tablescan but: “almost all the waits are on ‘db file sequential read’ and the disk read is 10 times the table blocks”.  Some further information supplied was that the tablespace was using ASSM and 16KB block size; the table had 272 columns (ouch!) and the Oracle version was 11.2.0.4.

Manuals

From time to time I read a question (or, worse, an answer) on OTN and wonder how someone could have managed to misunderstand some fundamental feature of Oracle – and then, as I keep telling people everyone should do – I re-read the manuals and realise that that sometimes the manuals make it really easy to come to the wrong conclusion.

Having nothing exciting to do on the plane to Bucharest today, I decided it was time to read the Concepts manual again – 12c version – to remind myself of how much I’ve forgotten. Since I was reading the mobi version on an iPad mini I can’t quote page numbers, but at “location 9913 of 16157″ I found the following text in a sidebar:

“LGWR can write redo log entries to disk before a transaction commits. The redo entries become permanent only if the transaction later commits.”

Not In CTAS

Everyone gets caught out some of the time with NOT IN.

NOT IN is not the opposite of IN.

This came up in a (fairly typical) question on OTN recently where someone had the task of “deleting 6M rows from a table of 18M”. A common, and perfectly reasonable, suggestion for dealing with a delete on this scale is to consider creating a replacement table holding the data you do want rather than deleting the data you don’t want.  In this case, however, the query for deleting the data looked like this:


DELETE FROM EI.CASESTATUS
     WHERE CASEID NOT IN (SELECT CASEID FROM DO.STG_CASEHEADER);

The suggested code for creating the kept data was this:

ASH

There was a little conversation on Oracle-L about ASH (active session history) recently which I thought worth highlighting – partly because it raised a detail that I had got wrong until Tim Gorman corrected me a few years ago.

Once every second the dynamic performance view v$active_session_history copies information about active sessions from v$session. (There are a couple of exceptions to the this rule – for example if a session has called dbms_lock.sleep() it will appear in v$session as state = ‘ACTIVE’, but it will not be recorded in v$active_session_history.) Each of these snapshots is referred to as a “sample” and may hold zero, one, or many rows.

12c MView refresh

Some time ago I wrote a blog note describing a hack for refreshing a large materialized view with minimum overhead by taking advantage of a single-partition partitioned table. This note describes how Oracle 12c now gives you an official way of doing something similar – the “out of place” refresh.

I’ll start by creating a matieralized view and creating a couple of indexes on the resulting underlying table; then show you three different calls to refresh the view. The materialized view is based on all_objects so it can’t be made available for query rewrite (ORA-30354: Query rewrite not allowed on SYS relations) , and I haven’t created any materialized view logs so there’s no question of fast refreshes – but all I intend to do here is show you the relative impact of a complete refresh.