The COMPLETION extension implements a table-valued function named "completion" that can be used to suggest completions of partially entered words during interactive SQL input. The completion table can be used to help implement tab-completion, for example.
The designed query interface is:
SELECT DISTINCT candidate COLLATE nocase FROM completion($prefix, $wholeline) ORDER BY 1;
The query above will return suggestions for the whole input word that begins with $prefix. The $wholeline parameter is all text from the beginning of the line up to the insertion point. The $wholeline parameter is used for context.
The $prefix parameter may be NULL, in which case the prefix is deduced from $wholeline. Or, the $wholeline parameter may be NULL or omitted if context information is unavailable or if context-aware completion is not desired.
The completion table might return the same candidate more than once, and it will return candidates in an arbitrary order. The DISTINCT keyword and the ORDER BY in the sample query above are added to make the answers unique and in lexicographical order.
The completion table is used to implement tab-completion in the command-line shell in conjunction with either the readline or linenoise input line editing packages for unix. See the https://sqlite.org/src/file/src/shell.c.in source file for example code. Search for "FROM completion" to find the relevant code sections.
Because the completion table is built into the command-line shell in order to provide for tab-completions, you can run test queries against the completion table directly in the command-line shell. Simply type a query such as the example shown above, filling in appropriate values for $prefix and $wholeline, and observe the output.
The completion table is designed for interactive use. It will return answers at a speed appropriate for human typing. No effort is made to be unusually efficient, so long as the response time is nearly instantaneous in a user interface.
As of this writing (2017-07-13), the completion virtual table only looks for SQL keywords, and schema, table, and column names. The context contained in $wholeline is completely ignored. Future enhancements will try to return new completions taken from function and pragma names and other sources, as well as consider more context. The completion table should be considered a work-in-progress.