Fast/Unload BLOB/CLOB processing considerations: Difference between revisions

Latest revision as of 15:54, 14 March 2016

As of version 4.3, Fast/Unload included the ability to operate on BLOB and CLOB (collectively called "Lob") fields, which were introduced with V6R1 of Model 204.

Note: Processing BLOB or CLOB fields that have the DEFAULT-VALUE attribute requires Fast/Unload version 4.6 or higher.

The following Fast/Unload 4.3 features support these fields:

NEW field declaration statements may have a WITH CLOB or WITH BLOB designation (for example, for creating a Lob field by concatenating "old" non-Lob fields). See NEW statement option for Lobs.
#functions may both accept arguments and produce results in excess of 255 bytes. See:
The contexts where FUEL %variables may contain strings longer than 255 bytes, the statements, #functions, and directives that allow string values longer than 255 bytes, and the contexts where Lob fields may be used are specified (see Contexts for long strings and Lobs).
A job statistic reports Table E page usage for each Lob field (Lob statistics).

The above features are discussed in the rest of this wiki page, at the end of which are two examples that use these features (Lob field examples).

Statement and #function modifications

The NEW statement and three #functions let you define Lob fields and work with strings longer than 255 bytes.

NEW statement option for Lobs

As of version 4.3, the NEW statement (NEW fieldname [WITH BLOB | CLOB\) lets you specify that the new field you are defining is either a BLOB or CLOB field. This is primarily useful for a UAI-type unload, allowing you to create values in the new field that are loaded by LAI as Lob occurrences.

The syntax is:

NEW fieldname WITH BLOB | CLOB

Note: Version 4.3 also introduced a change to the default attributes that are assigned to fields defined with NEW. As of version 4.3, the default attributes are NFRV, NKEY, NCOD, UPDATE IN PLACE (formerly, they were FRV, KEY, CODED, UPDATE AT END).

See Creating a NEW Lob field for an example that uses a Lob option.

#CONCAT supports long string arguments and result

The arguments of #CONCAT may be string values that exceed 255 bytes in length (as the contents of %variables or Lob fields).

The result of #CONCAT may be a string longer than 255 bytes.

Prior to version 4.3 of Fast/Unload, the maximum length of an argument was 255 bytes, and if the concatenation of the arguments exceeded 255 bytes, the FUEL program was terminated.

See Creating a NEW Lob field for an example that uses this #function with a long string value.

#LEN supports a long string argument

The first argument of #LEN may be a string value that exceeds 255 bytes in length (as the content of a %variable or Lob field).

Prior to version 4.3 of Fast/Unload, the maximum length of an argument was 255 bytes.

See Structured unload of Lob field for an example that uses this #function with a long string value.

#SUBSTR supports a long string argument and result

The first argument of #SUBSTR may be a string value that exceeds 255 bytes in length (as the content of a %variable or Lob field).

The result of #SUBSTR may be a string longer than 255 bytes.

Prior to to version 4.3 of Fast/Unload, the maximum length of an argument was 255 bytes.

See Structured unload of Lob field for an example that uses this #function with a long string value.

Contexts for long strings and Lobs

The version 4.3 Fast/Unload accommodations for Lob fields included allowing %variables to contain strings longer than 255 bytes and specifying the contexts that allow such strings and Lob fields.

%Variables containing strings longer than 255

The value of a %variable may be a string longer than 255 bytes. This can arise as the result of:

%v1 = %v2	Assignment from another %variable that contains a string longer than 255 bytes
%v = fld	Assignment from a Lob field
%v = `#SUBSTR(...)`	Assignment from a substring of a string value longer than 255 bytes
%v = `#CONCAT(...)`	Assignment from the concatenation of strings, whose lengths total more than 255 bytes

Permitted use of long string values specifies the contexts in which a %variable may be used if it contains a string longer than 255 bytes.

Permitted use of long string values

A long string value may be used in the following contexts:

As an argument of #CONCAT
As the argument of #LEN
As the first argument of #SUBSTR
As the right-hand side of an assignment to a %variable
As the right-hand side of a CHANGE or ADD[C] statement, when the field on the left-hand side is a Lob

If the value of a %variable is used in any other context, and it is a string longer than 255 bytes, the FUEL program is terminated. For example, the following program creates one line of output, because the PUT statement does not allow a %variable containing a string longer than 255:

OPEN MYFILE %X = #LEFT('ABC', 150, 'Z') PUT %X /* Length is 150 OUTPUT %X = #CONCAT(%X, %X) /* Length is 300 PUT %X /* FUEL program will be cancelled here OUTPUT FOR EACH RECORD /* Make it a legal FUEL program END FOR

Other examples of contexts prohibiting a %variable containing a string longer than 255 bytes include arithmetic expressions, comparisons in the IF statement, and more.

Note that, since the EXISTS and MISSING clauses of the IF and ELSEIF statements do not reference the value of a %variable, you may use them to test a %variable even if it contains a string longer than 255 bytes. That is, the following statement is acceptable in all cases:

IF %S MISSING THEN /* OK even if #LEN(%S) > 255

Permitted use of Lobs

The value of a Lob field may only be used in the contexts discussed above that allow a string longer than 255 bytes, even if the actual length of the Lob field occurrence does not exceed 255. Use of a Lob field in an invalid context causes the compilation of the FUEL program to fail; it never begins execution.

There are four contexts in which any field, Lob or not, may be referenced:

The UNLOAD(C) statement
The EXISTS and MISSING clauses of an IF/ELSEIF statement
The #IF/#ELSEIF directives
Preceding the number sign (#) "qualifier," which specifies the number of occurrences of the field.
For example, the following statement is valid for any type of field:

FOR I FROM 1 TO BLOB(#) /* OK for any field

Lob statistics

If you display field statistics in the Fast/Unload job statistics, the total number of pages used in Table E is shown for each Lob field on the second line of the field's display.

Note that the length statistics given for a Lob field, just like other fields, is based on the field occurrence values: in this case, the number of bytes in Table E used by each field occurrence value (that is, unused bytes in Table E pages are not included in the length statistics).

The Table B usage for a Lob field is:

27 bytes for a non-preallocated Lob field occurrence (in addition to the overhead, as usual, for a count byte and field code)
28 bytes for a preallocated Lob field occurrence

For more information about the field statistics, see FSTATS [AVGTOT | MINMAX\.

Lob field examples

Creating a NEW Lob field

The following example unloads file PRODFILE such that, when it is reloaded, all occurrences of field COMMENT are combined into a single Lob field named ALLCOMMENTS:

OPEN PRODFILE UAI OINDEX NEW ALLCOMMENTS WITH BLOB FOR EACH RECORD IF COMMENT EXISTS THEN %X = /* Initialize BLOB value FOR I FROM 1 TO COMMENT(#) %X = #CONCAT(%X, COMMENT) DELETE COMMENT END FOR ADD ALLCOMMENTS = %X END IF UNLOAD END FOR

Note that the first occurrence of COMMENT is used in each iteration of the FOR I loop. When this occurrence is deleted at the tail of the loop, the occurrence after it becomes the first occurrence on the next iteration.

Structured unload of Lob field

The following example unloads file PRODFILE, creating one output record for each 255 bytes (the maximum for a PUT statement) of the Lob field named ALLCOMMENTS:

OPEN PRODFILE FOR EACH RECORD PUT '* ' PUT CUSTOMER_ID OUTPUT IF ALLCOMMENTS EXISTS THEN %COM = ALLCOMMENTS %LENGTH = #LEN(ALLCOMMENTS) %I = 1 %LIM = %LENGTH - 254 REPEAT IF +%I >= %LIM THEN LEAVE REPEAT END IF %X = #SUBSTR(%COM, %I, 255) PUT %X OUTPUT %I = %I + 255 END REPEAT %X = #SUBSTR(%COM, %I) PUT %X OUTPUT END IF END FOR

Important notes:

The plus sign (+) in IF +%I >= %LIM is very important — otherwise a string comparison is done, which is not correct. For example, if the length is 1,000,000, the first 255 bytes would be unloaded and the final PUT will fail, because then the length of %X would be 1,000,000-254.

The above approach is vastly superior to an approach that uses something like %COM = $SUBSTR(#COM, 256) to repeatedly remove the first 255 bytes, because that would involve unnecessary copying (approximately the square of the number of bytes in each field).