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The document discusses C++ and its history and features. It describes C++ as an extension of C with object-oriented features like classes. It provides information on the creator of C++, Bjarne Stroustrup, and the evolution of C++ since its introduction as a way to add object-oriented programming to C. It also includes sample C++ code demonstrating the use of arrays and includes from the Boost libr
Index and Table AccessSeq ScanThe Seq Scan operation scans the entire relation (table) as stored on disk (like TABLE ACCESS FULL). Index ScanThe Index Scan performs a B-tree traversal, walks through the leaf nodes to find all matching entries, and fetches the corresponding table data. It is like an INDEX RANGE SCAN followed by a TABLE ACCESS BY INDEX ROWID operation. See also Chapter 1, “Anatomy o
PostgreSQL には 通常の DISTINCT ではない DISTINCT ON というものがあります。 次のようなテーブルがあるとします。 db1=# SELECT * FROM distinct_test; field1 | field2 | field3 --------+--------+-------- 1 | 4 | 1 1 | 4 | 2 2 | 2 | 1 2 | 2 | 2 3 | -1 | 1 3 | -1 | 2 (6 rows) まずは通常の DISTINCT です。 db1=# SELECT DISTINCT field1, field2 FROM distinct_test; field1 | field2 --------+-------- 1 | 4 2 | 2 3 | -1 (3 rows) この場合、列に指定した field1, field2 の順
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