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40 lines
1.6 KiB
40 lines
1.6 KiB
11 months ago
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Indexes
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=======
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* An index is a data structure (usually a B-tree) for fast column lookups.
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* You don't need to (and probably shouldn't) put an index on every column, but if for example a column is used in the WHERE clause of an SQL statement, you should consider adding an index to that column.
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* If a table is not indexed, queries may go slow because the database may have to do a full table scan (i.e. linear search).
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CREATE TABLE RandomData(
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ID INTEGER PRIMARY KEY,
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Num FLOAT NOT NULL
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);
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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SELECT * FROM RandomData;
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INSERT INTO RandomData(Num) SELECT a.Num/b.Num AS Num FROM RandomData AS a JOIN RandomData AS b ON a.ID != b.ID;
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INSERT INTO RandomData(Num) SELECT a.Num/b.Num AS Num FROM RandomData AS a JOIN RandomData AS b ON a.ID != b.ID;
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INSERT INTO RandomData(Num) SELECT a.Num/b.Num AS Num FROM RandomData AS a JOIN RandomData AS b ON a.ID != b.ID;
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SELECT COUNT(*) FROM RandomData;
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SELECT * FROM RandomData WHERE ID=1234567; /* SQLite automatically puts an index on PRIMARY KEY and UNIQUE columns */
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SELECT * FROM RandomData WHERE Num>=.998 AND Num<=.999; /* Not indexed; it takes a second. */
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CREATE INDEX Idx_RandomData_Num ON RandomData(Num); /* Naming convention for an index: Idx_TableName_ColumnName; */
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SELECT * FROM RandomData WHERE Num>=.998 AND Num<=.999;
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DROP INDEX Idx_RandomData_Num;
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SELECT * FROM RandomData LIMIT 10;
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