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In this lesson, we're going to
study the concept of a database
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block in Oracle.
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So a database block is
the smallest atomic unit
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of space in an Oracle database.
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If you've ever heard the
concept of an operating system
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block or a file
system block, this
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is sort of analogous to that.
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So, when Oracle stores
space or reads space,
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it never reads less than
the size of an Oracle block.
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So, even if it only needs one
row that's maybe 100 bytes,
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it still reads the
entire block into memory.
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So database blocks
are stored on disk
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in the form of extents and
segments and then data files.
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But the blocks are the
smallest part of that.
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And so a database block is
read into a memory buffer
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or a buffer in the cache.
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So we have something
like the SGA
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and the buffer cache
that's present there.
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And so the database
blocks are actually
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read into those buffers.
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The database blocks size is
defined by the DB block size
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parameter in the database.
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And this is defined at creation.
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And, once the database is
created with a certain block
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size, it cannot be changed.
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So your only option if you
want to change the block
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size of the entire database
is to export all of the data
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out, recreate the database
with the new block size,
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and then import
the data back in.
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So the database
block sizes are going
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to match the buffer sizes
because a block fits perfectly
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into a buffer, if you will.
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So if we had an 8K block
size for the database,
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that means our buffers are
going to be 8K as well.
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And, since the block size cannot
be changed after creation,
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it's important that we know how
to properly size our database
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block, so what block size is
available to us and what is
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most advisable for
our certain situation.
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So, to look at the anatomy
of a database block a little,
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let's say this is
one database block.
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Database block consists of
header information, free space,
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and used space.
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So the header
information is going
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to be a small amount that's
used for the block that
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will have information,
such as something called
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the DBA or the
Data Block Address,
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where it's located
physically on disk,
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and some other information
about columns and tables
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that are within the block.
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The next section
is the free space.
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So a database block will
have space that's used
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and space that's not.
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And so the free
space will be what
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we constitute as the open and
available space for new rows.
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The used space
will be the actual
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rose on disk within the block.
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So, if we see here, we could
look at this as a row of data.
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And so the used space is going
to be filled with row data,
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and then the free space
will be available for use.
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So this line actually moves
within every database block.
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As a block begins to fill
up with more and more rows
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of data, this free space
gets less and less.
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So we said that this
sizing of a database block
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was very important because
it can't be changed.
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So what are our possibilities?
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Well, we can have a database
block size between 2 kilobytes
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and 32 kilobytes.
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So how do we decide
what is best?
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Well, let's start with
the middle which is 8K.
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So the sizing possibilities are
2K to 32k in a doubling order,
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if you will.
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2, 4, 8, 16, and 32.
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Those are the five possibilities
for our block size.
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The default is 8K.
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And, for most situations,
that's going to be a good size.
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Let's look at the top end.
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So the smallest block
possibility is 2K.
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2K will not be used
all that often.
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You won't generally see
it being used because it's
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a very small block size.
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The next is more
common and that is 4K.
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So a 4K block size is going
to be ideal for situations
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where the database does
something like, OLTP,
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a lot of online
transaction processing.
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So this is many small
discrete transactions.
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So we could think of an OLTP as
something like an order entry
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system, maybe an online
store of some kind
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because you go and
you have a page that
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shows you the possible
things that you can purchase,
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then you create a shopping cart.
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And, once you filled all that
information about your order,
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you click Submit and that's
a single discrete transaction
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