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array header, and to compute sizing and alignment of array elements the same way normal tuple access operations do --- viz, using the tupmacs.h macros att_addlength and att_align. This makes the world safe for arrays of cstrings or intervals, and should make it much easier to write array-type-polymorphic functions; as examples see the cleanups of array_out and contrib/array_iterator. By Joe Conway and Tom Lane.
50 lines
2.1 KiB
Plaintext
50 lines
2.1 KiB
Plaintext
Array iterator functions, by Massimo Dal Zotto <dz@cs.unitn.it>
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Copyright (C) 1999, Massimo Dal Zotto <dz@cs.unitn.it>
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This software is distributed under the GNU General Public License
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either version 2, or (at your option) any later version.
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This loadable module defines a new class of functions which take
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an array and a scalar value, iterate a scalar operator over the
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elements of the array and the value, and compute a result as
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the logical OR or AND of the iteration results.
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For example array_int4eq returns true if some of the elements
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of an array of int4 is equal to the given value:
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array_int4eq({1,2,3}, 1) --> true
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array_int4eq({1,2,3}, 4) --> false
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If we have defined T array types and O scalar operators we can
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define T x O x 2 array functions, each of them has a name like
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"array_[all_]<basetype><operation>" and takes an array of type T
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iterating the operator O over all the elements. Note however
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that some of the possible combination are invalid, for example
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the array_int4_like because there is no like operator for int4.
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We can then define new operators based on these functions and use
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them to write queries with qualification clauses based on the
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values of some of the elements of an array.
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For example to select rows having some or all element of an array
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attribute equal to a given value or matching a regular expression:
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create table t(id int4[], txt text[]);
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-- select tuples with some id element equal to 123
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select * from t where t.id *= 123;
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-- select tuples with some txt element matching '[a-z]'
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select * from t where t.txt *~ '[a-z]';
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-- select tuples with all txt elements matching '^[A-Z]'
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select * from t where t.txt[1:3] **~ '^[A-Z]';
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The scheme is quite general, each operator which operates on a base type
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can be iterated over the elements of an array. It seem to work well but
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defining each new operator requires writing a different C function.
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This is tedious, and error-prone since one must take care that the correct
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datatypes are associated with the selected underlying function.
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Can anyone suggest a better and more portable way to do it ?
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See also array_iterator.sql for an example on how to use this module.
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