@   origid INTEGER REFERENCES blob, -- check-in holding propagated tag
@   value TEXT,                     -- Value of the tag.  Might be NULL.
@   mtime TIMESTAMP,                -- Time of addition or removal
@   rid INTEGER REFERENCE blob,     -- Artifact tag is applied to
@   UNIQUE(rid, tagid)
@ );
@ CREATE INDEX tagxref_i1 ON tagxref(tagid, mtime);
@
@ -- When a hyperlink occurs from one artifact to another (for example
@ -- when a check-in comment refers to a ticket) an entry is made in
@ -- the following table for that hyperlink.  This table is used to
@ -- facilitate the display of "back links".
@ --
@ CREATE TABLE backlink(
@   target TEXT,           -- Where the hyperlink points to
@   srctype INT,           -- 0: check-in  1: ticket  2: wiki
@   srcid INT,             -- rid for checkin or wiki.  tkt_id for ticket.
@   mtime TIMESTAMP,       -- time that the hyperlink was added
@   UNIQUE(target, srctype, srcid)
@ );
@ CREATE INDEX backlink_src ON backlink(srcid, srctype);
@
@ -- Template for the TICKET table
@ --
@ -- NB: when changing the schema of the TICKET table here, also make the
@ -- same change in tktsetup.c.
@ --
@ CREATE TABLE ticket(