## SQLite Expert Extension

This folder contains code for a simple system to propose useful indexes
given a database and a set of SQL queries. It works as follows:

  1. The user database schema is copied to a temporary database.

  1. All SQL queries are prepared against the temporary database. I
     Information regarding the WHERE and ORDER BY clauses, and other query
     features that affect index selection, are recorded.

  1. The information gathered in step 2 is used to create (possibly a large
     number of) candidate indexes.

  1. A subset of the data in the user database is used to generate statistics
     for all existing indexes and the candidate indexes generated in step 3
     above.

  1. The SQL queries are prepared a second time. If the planner uses any
     of the indexes created in step 3, they are recommended to the user.

# C API

The SQLite expert C API is defined in sqlite3expert.h. Most uses will proceed
as follows:

  1. An sqlite3expert object is created by calling **sqlite3\_expert\_new()**.
     A database handle opened by the user is passed as an argument.

  1. The sqlite3expert object is configured with one or more SQL statements
     by making one or more calls to **sqlite3\_expert\_sql()**. Each call may
     specify a single SQL statement, or multiple statements separated by
     semi-colons.
  
  1. Optionally, the **sqlite3\_expert\_config()** API may be used to 
     configure the size of the data subset used to generate index statistics.
     Using a smaller subset of the data can speed up the analysis.

  1. **sqlite3\_expert\_analyze()** is called to run the analysis.

  1. One or more calls are made to **sqlite3\_expert\_report()** to extract
     components of the results of the analysis.

  1. **sqlite3\_expert\_destroy()** is called to free all resources.

Refer to comments in sqlite3expert.h for further details.

# sqlite3_expert application

The file "expert.c" contains the code for a command line application that
uses the API described above. It can be compiled with (for example):

<pre>
  gcc -O2 sqlite3.c expert.c sqlite3expert.c -o sqlite3_expert
</pre>

Assuming the database is named "test.db", it can then be run to analyze a
single query:

<pre>
  ./sqlite3_expert -sql &lt;sql-query&gt; test.db
</pre>

Or an entire text file worth of queries with:

<pre>
  ./sqlite3_expert -file &lt;text-file&gt; test.db
</pre>

By default, sqlite3_expert generates index statistics using all the data in
the user database. For a large database, this may be prohibitively time
consuming. The "-sample" option may be used to configure sqlite3_expert to
generate statistics based on an integer percentage of the user database as
follows:

<pre>
  # Generate statistics based on 25% of the user database rows:
  ./sqlite3_expert -sample 25 -sql &lt;sql-query&gt; test.db

  # Do not generate any statistics at all:
  ./sqlite3_expert -sample 0 -sql &lt;sql-query&gt; test.db
</pre>