Active 10 years, 3 months ago. Viewed 3k times. So: what does one normally use writefds for? Improve this question. Django Django 1 1 gold badge 7 7 silver badges 14 14 bronze badges. Add a comment. Active Oldest Votes. EDIT From the standard : The pselect function shall examine the file descriptor sets whose addresses are passed in the readfds, writefds , and errorfds parameters to see whether some of their descriptors are ready for reading, are ready for writing , or have an exceptional condition pending, respectively.
Improve this answer. Community Bot 1 1 1 silver badge. Ok thnx, this was the answer I was looking for. By the way, why do your write select 2 instead of select , what does it mean? Django It's the manual section. Ok, I see. I have been searching these pages to find out what writefds exactly means a. Is it somewhere in these pages? Yeah ok, I saw that, but where did you get that juicy detail that a writable state means that it will send at least one byte? Django Well if it's writable, it has to send something.
Working with a huge set of functional dependencies can cause unnecessary added computational time. This is where the canonical cover comes into play. A canonical cover of a set of functional dependencies F is a simplified set of functional dependencies that has the same closure as the original set F.
Important definitions: Attention reader! Extraneous attributes: An attribute of a functional dependency is said to be extraneous if we can remove it without changing the closure of the set of functional dependencies.
Canonical cover: A canonical cover of a set of functional dependencies F such that ALL the following properties are satisfied:. Finding Canonical Cover Algorithm to compute canonical cover of set F: repeat 1. Use the union rule to replace any dependencies in and with. Find a functional dependency with an extraneous attribute either in or in.
If an extraneous attribute is found, delete it from. None of the attributes in the left or right side of any functional dependency is extraneous Checked by applying definition of extraneous attributes on every functional dependency. Hence, the canonical cover is equal to F. Note: There can be more than one canonical cover of a set F of functional dependencies. How to check whether a set of f. This means, we need to find out whether F canonically covers G, G canonically covers F, or none of the two canonically cover the other.
To find out, we follow the following steps: Create a singleton right hand side. This means, the attributes to the right side of the f. Consider any functional dependency XY Z. If X in itself can determine Z, then the attribute Y is extraneous and can be removed. As we can see, the occurrence of extraneous attributes is possible only in those functional dependencies where there are more than one attributes in the LHS.
So, consider the functional dependency AB C. To fix this problem , we need to break this table into two: one to hold information about the student and the other to hold information about the program. However we still need to leave an FK in the student table so that we can identify which program the student is enrolled in. This rule suggests that if two tables are separate, and the PK is the same, you may want to consider putting them together.
Some database administrators DBA might choose to keep these tables separated for a couple of reasons. One, each table describes a different entity so the entities should be kept apart. Decomposition is the reverse of the Union rule. If you have a table that appears to contain two entities that are determined by the same PK, consider breaking them up into two tables.
A dependency diagram, shown in Figure A non-normalized table is one that has data redundancy in it. Union : a rule that suggests that if two tables are separate, and the PK is the same, consider putting them together. Skip to content Main Body.
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