Mike Matrigali created DERBY-5799:
-------------------------------------
Summary: add more info on when Derby chooses 32k page size for
tables.
Key: DERBY-5799
URL: https://issues.apache.org/jira/browse/DERBY-5799
Project: Derby
Issue Type: Improvement
Components: Documentation
Affects Versions: 10.8.2.2
Reporter: Mike Matrigali
Priority: Minor
Enhance the following documentation to explain all cases when Derby will
default a table to 32k.
current doc:
http://db.apache.org/derby/docs/10.8/ref/rrefproper40688.html
1) better describe what "long" means. Originally this was clear as it meant
just datatypes with
the "long" qualifier. Now it also includes Blob and Clob columns. Not sure
if we should list
all the types, or just something like long, Blob and/or Clob.
2) Derby will also choose 32k if the approximate length of the columns declared
at create time is greater than 4k. Approximate is used as each different
datatype may or may not be fixed length
when stored on the page. Character data may take 1, 2, or 3 bytes per
character, obviously
variable length data like varchar can't be known at create time, ...
I don't think we should try to document exact derby algorithm, rather doc that
Derby will default
to what it thinks should be the best size of either 4k or 32k unless user
chooses to override. I
do think it is useful to doc that Derby will include both existence of long
columns and the estimate
length of the row.
Here some comments from Property.java that describes constants used, if these
constants every change then documentation would change also:
/**
* The default page size to use for tables that contain a long column.
**/
public static final String PAGE_SIZE_DEFAULT_LONG = "32768";
/**
* The bump threshold for pages sizes for create tables
* If the approximate column sizes of a table is greater than this
* threshold, the page size for the tbl is bumped to PAGE_SIZE_DEFAULT_LONG
* provided the page size is not already specified as a property
**/
public static final int TBL_PAGE_SIZE_BUMP_THRESHOLD = 4096;
/**
* The bump threshold for pages size for index.
* If the approximate key columns of an index is greater than this
* threshold, the page size for the index is bumped to PAGE_SIZE_DEFAULT_LON
G
* provided the page size is not already specified as a property
**/
public static final int IDX_PAGE_SIZE_BUMP_THRESHOLD = 1024;
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