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This cause of the ORA-01555 error is hard to eliminate entirely, but it is rare anyway, as the circumstances under which it occurs do not happen frequently (at least not in Oracle8i and above anymore). We have already discussed the block cleanout mechanism, but to summarize, it is the process whereby the next session to access a block after it has been modified may have to check to see if the transaction that last modified the block is still active. Once the process determines that the transaction is not active, it cleans out the block so that the next session to access it does not have to go through the same process again. To clean out the block, Oracle determines the undo segment used for the previous transaction (from the blocks header) and then determines if the undo header indicates whether it has been committed or not. This confirmation is accomplished in one of two ways. One way is that Oracle can determine that the transaction committed a long time ago, even though its transaction slot has been overwritten in the undo segment transaction table. The other way is that the COMMIT SCN is still in the transaction table of the undo segment, meaning the transaction committed a short time ago, and its transaction slot hasn t been overwritten. To receive the ORA-01555 error from a delayed block cleanout, all of the following conditions must be met: A modification is made and COMMITed, and the blocks are not cleaned out automatically (e.g., it modified more blocks than can fit in 10 percent of the SGA block buffer cache). These blocks are not touched by another session and will not be touched until our unfortunate query (displayed shortly) hits it.
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CHAPTER 9 REDO AND UNDO
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A long-running query begins. This query will ultimately read some of those blocks from earlier. This query starts at SCN t1, the read consistent SCN it must roll back data to in order to achieve read consistency. The transaction entry for the modification transaction is still in the undo segment transaction table when we began. During the query, many commits are made in the system. These transactions do not touch the blocks in question (if they did, then we wouldn t have the impending problem). The transaction tables in the undo segments roll around and reuse slots due to the high degree of COMMITs. Most important, the transaction entry for the original modification transaction is cycled over and reused. In addition, the system has reused undo segment extents, so as to prevent a consistent read on the undo segment header block itself. Additionally, the lowest SCN recorded in the undo segment now exceeds t1 (it is higher than the read-consistent SCN of the query), due to the large number of commits. When our query gets to the block that was modified and committed before it began, it is in trouble. Normally, it would go to the undo segment pointed to by the block and find the status of the transaction that modified it (in other words, it would find the COMMIT SCN of that transaction). If the COMMIT SCN is less than t1, our query can use this block. If the COMMIT SCN is greater than t1, our query must roll back that block. The problem is, however, that our query is unable to determine in this particular case if the COMMIT SCN of the block is greater than or less than t1. It is unsure as to whether it can use that block image or not. The ORA-01555 error then results. To see this, we will create many blocks in a table that need to be cleaned out. We will then open a cursor on that table and allow many small transactions to take place against some other table not the table we just updated and opened the cursor on. Finally, we will attempt to fetch the data for the cursor. Now, we know that the data required by the cursor will be OK we should be able to see all of it since the modifications to the table would have taken place and been committed before we open the cursor. When we get an ORA-01555 error this time, it will be because of the previously described problem. To set up for this example, we ll use The 2MB UNDO_SMALL undo tablespace (again). A 4MB buffer cache, which is enough to hold about 500 blocks. This is so we can get some dirty blocks flushed to disk to observe this phenomenon. Before we start, we ll create the big table we ll be querying: ops$tkyte@ORA10G> create table big 2 as 3 select a.*, rpad('*',1000,'*') data 4 from all_objects a; Table created. ops$tkyte@ORA10G> exec dbms_stats.gather_table_stats( user, 'BIG' ); PL/SQL procedure successfully completed.
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