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/**

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@mainpage evsql

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@author Tero Marttila

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@section Introduction

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Evsql is a C-language SQL library designed for use with libevent, and primarily PostgreSQL's libpq.

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Evsql was born as a result of wanting to use a SQL database from within a libevent application, and discovering

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libpq's asynchronous API. Since the libpq API is somewhat cumbersome to use, I wrote a separate interface that

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drives libpq using libevent and makes writing asynchronous SQL clients a lot easier - plus adding some conveniance

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for parametrized queries and such along the way.

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The evsql API doesn't expose the underlying database library's API, although since the only currently existing

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implementation is libpq, this should really be thought of as a generically-named PostgreSQL library rather than a

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database-agnostic API...

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@section Usage

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Include the top-level evsql.h header, making sure you also have the evpq and lib modules available.

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@section Connecting

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Evsql sessions are represented using an opaque struct, called simply evsql. Use the \ref evsql_ evsql_new_ function corresponding to your

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database engine (PostgreSQL -> evsql_new_pq()) to allocate this handle. It is valid for use immediately, although the 

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initial connection may not yet be complete.

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There is an evsql_close function, but it is currently not implemented.

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@section Transactions

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Evsql supports both non-transactional queries and transactional queries. A transaction is allocated its own dedicated

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connection which it can use for its queries without being interfered by other queries/transactions. Evsql takes care of

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sending the initial "BEGIN TRANSACTION" query, and provides evsql_trans_commit()/evsql_trans_abort() functions to send the

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"COMMIT TRANSACTION" and "ROLLBACK TRANSACTION" queries.

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Note, however, that transactions can only handle one outstanding query at a time, whereas transactionless queries can

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be enqueued by evsql itself.

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@section Querying

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There is a single evsql_query function used for both transactional and non-transactional queries; you can pass NULL

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as the trans argument.

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The given callback function is invoked once the query has been processed and the result is available, or the query

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failed.

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The important distinction between transactional and non-transactional queries is that transactions only support one

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outstanding query at a time, meaning that you must wait for your callback to be invoked before calling evsql_query

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again for the transaction. Non-transactional queries are sent using an idle connection, and will be enqueued for later

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execution if no idle connections are available.

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evsql_query returns a handle that can be passed to evsql_query_abort to abort the query, ensuring that the callback

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given to evsql_query is not invoked, and any associated resources released.

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@section Parametrized Queries

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Evsql also provides functions to send parametrized queries, the behaviour of evsql_query explained above applies as

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well. 

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The first of these is evsql_query_params, which takes the parametrized SQL command and a struct containing the

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parameter types and values as arguments.

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The second of these is evsql_query_exec, which takes a evsql_query_info struct containing the parametrized SQL command

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and the parameter types, and the parameter values themselves as a list of variable arguments (of the correct type!).

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@section API Reference

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The entire API is defined in the top-level evsql.h header.

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*/