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What is row migration Row migration is when a row is forced to leave the block it was created on because it grew too large to fit on that block with the rest of the rows. To illustrate row migration, we start with a block that looks like Figure 10-3.

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Figure 10-3. Data block before update Approximately one-seventh of the block is free space. However, we would like to more than double the amount of space used by row 4 via an UPDATE (it currently consumes one-seventh of the block). In this case, even if Oracle coalesced the space on the block as shown in Figure 10-4, there is still insufficient room to double the size of row 4, because the size of the free space is less than the current size of row 4.

fold_left eval env stmts | IfThen (e, stmt) -> if evalE env e <> 0 then eval env stmt else env | IfThenElse (e, stmt1, stmt2) -> if evalE env e <> 0 then eval env stmt1 else eval env stmt2 | Print e -> print_int (evalE env e); env With these at hand, continuing the same interactive session, you can now evaluate the sample Kitty program: > match parseText sample with | Prog stmts -> eval Mapempty (Seq stmts) |> ignore;; 4950 val it : unit = ().

Figure 10-4. Data block as it would appear after coalescing free space If the row fit into the coalesced space, it would have happened. This time, however, Oracle will not perform this coalescing and the block will remain as it is. Since row 4 would have to span more than one block if it stayed on this block, Oracle will move, or migrate, the row. However, Oracle cannot just move the row; it must leave behind a forwarding address. There may be indexes that physically point to this address for row 4. A simple update will not modify the indexes as well.

Note There is a special case with partitioned tables that a rowid, the address of a row, will change. We will look

If necessary, you can also compile the AST, the lexer, and the parser together into a DLL or as part of an EXE: fsc a o KittySyntax.dll kittyAst.fs kittyParser.fs kittyLexer.fs

at this case in 13 Partitioning. Additionally, other administrative operations such as FLASHBACK TABLE and ALTER TABLE SHRINK may change rowids assigned to rows as well.

Therefore, when Oracle migrates the row, it will leave behind a pointer to where the row really is. After the update, the blocks might look as shown in Figure 10-5.

There is one final case of parsing that is common when working with binary data That is, say you want to work with a format that is conceptually relatively easy to parse and generate (such as a binary format) but where the process of actually writing the code to crack and encode the format is somewhat tedious In this section, we cover a useful set of techniques to write readers and writers for binary data quickly and reliably As the running example, we will show a set of pickling (also called marshalling) and unpickling combinators to generate and read a binary format of our own design The combinators can easily be adapted to work with existing binary formats such as those used for network packets Picklers and unpicklers for different data types will be function values that have signatures as follows: type outstate = SystemIO.

Figure 10-5 Migrated row depiction So, a migrated row is a row that had to move from the block it was inserted into onto some other block Why is this an issue Your application will never know; the SQL you use is no different It only matters for performance reasons If you go to read this row via an index, the index will point to the original block That block will point to the new block Instead of doing the two or so I/Os to read the index plus one I/O to read the table, you ll need to do yet one more I/O to get to the actual row data In isolation, this is no big deal you won t even notice it However, when you have a sizable percentage of your rows in this state, with many users accessing them, you ll begin to notice this side effect.