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84 lines
3.0 KiB
Markdown
84 lines
3.0 KiB
Markdown
## SQLite Expert Extension
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This folder contains code for a simple system to propose useful indexes
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given a database and a set of SQL queries. It works as follows:
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1. The user database schema is copied to a temporary database.
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1. All SQL queries are prepared against the temporary database.
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Information regarding the WHERE and ORDER BY clauses, and other query
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features that affect index selection are recorded.
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1. The information gathered in step 2 is used to create candidate
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indexes - indexes that the planner might have made use of in the previous
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step, had they been available.
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1. A subset of the data in the user database is used to generate statistics
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for all existing indexes and the candidate indexes generated in step 3
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above.
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1. The SQL queries are prepared a second time. If the planner uses any
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of the indexes created in step 3, they are recommended to the user.
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# C API
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The SQLite expert C API is defined in sqlite3expert.h. Most uses will proceed
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as follows:
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1. An sqlite3expert object is created by calling **sqlite3\_expert\_new()**.
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A database handle opened by the user is passed as an argument.
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1. The sqlite3expert object is configured with one or more SQL statements
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by making one or more calls to **sqlite3\_expert\_sql()**. Each call may
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specify a single SQL statement, or multiple statements separated by
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semi-colons.
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1. Optionally, the **sqlite3\_expert\_config()** API may be used to
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configure the size of the data subset used to generate index statistics.
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Using a smaller subset of the data can speed up the analysis.
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1. **sqlite3\_expert\_analyze()** is called to run the analysis.
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1. One or more calls are made to **sqlite3\_expert\_report()** to extract
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components of the results of the analysis.
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1. **sqlite3\_expert\_destroy()** is called to free all resources.
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Refer to comments in sqlite3expert.h for further details.
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# sqlite3_expert application
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The file "expert.c" contains the code for a command line application that
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uses the API described above. It can be compiled with (for example):
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<pre>
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gcc -O2 sqlite3.c expert.c sqlite3expert.c -o sqlite3_expert
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</pre>
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Assuming the database is named "test.db", it can then be run to analyze a
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single query:
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<pre>
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./sqlite3_expert -sql <sql-query> test.db
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</pre>
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Or an entire text file worth of queries with:
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<pre>
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./sqlite3_expert -file <text-file> test.db
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</pre>
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By default, sqlite3\_expert generates index statistics using all the data in
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the user database. For a large database, this may be prohibitively time
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consuming. The "-sample" option may be used to configure sqlite3\_expert to
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generate statistics based on an integer percentage of the user database as
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follows:
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<pre>
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# Generate statistics based on 25% of the user database rows:
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./sqlite3_expert -sample 25 -sql <sql-query> test.db
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# Do not generate any statistics at all:
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./sqlite3_expert -sample 0 -sql <sql-query> test.db
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</pre>
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