What is use of Virtual table auto generated tables

Question

I was doing some debugging with sql queries, executed following to create an Virtual Table:

sqlite> CREATE VIRTUAL TABLE email_fts USING fts3(subject, body, "to", "from");

& checked schema where it showing additional 3 table references to created Virtual table.

sqlite> .schema

CREATE VIRTUAL TABLE email_fts USING fts3(subject, body, "to", "from");

CREATE TABLE 'email_fts_content'(docid INTEGER PRIMARY KEY, 'c0subject', 'c1body', 'c2to', 'c3from');

CREATE TABLE 'email_fts_segments'(blockid INTEGER PRIMARY KEY, block BLOB);

CREATE TABLE 'email_fts_segdir'(level INTEGER,idx INTEGER,start_block INTEGER,leaves_end_block INTEGER,end_block INTEGER,root BLOB,PRIMARY KEY(level, idx));

Anyone please explain:

• what is use of fits_content/segments/segdir,
• who use these.
• Purpose of use surely be related to searching but how these related to each other.

Answer 1

This is documented in the documentation:

For each FTS virtual table in a database, three to five real (non-virtual) tables are created to store the underlying data. These real tables are called "shadow tables".
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