Keep the record count and total size in a totals table (closes #146)

info counted the records and added up their sizes with a query that
reads every record, which takes minutes on a large database. The
schema now has a totals table of one row, kept current by three
triggers on files, in the transaction of each insert, delete and size
update. info, scan's load phase and the read phases of report and
trees read that row instead of counting over files. scan writes files
with the same statements as before. Each of those writes now fires a
trigger and is slower, so scan keeps SQLite's temporary data in
memory, which wins back part of that time.

Model: opus-5-5

README.md

@@ -393,6 +393,20 @@ All four subcommands operate on a single SQLite database file:
         content    BLOB NOT NULL      -- raw 32-byte SHA-256; whole file under 100 MiB, else samples
     ) WITHOUT ROWID;
     CREATE INDEX files_signature ON files (size, head, tail, content);
+    CREATE TABLE totals (
+        records INTEGER NOT NULL,  -- number of records in files
+        size    INTEGER NOT NULL   -- sizes of all records in files added up
+    );
+    INSERT INTO totals (records, size) VALUES (0, 0);
+    CREATE TRIGGER totals_after_insert AFTER INSERT ON files BEGIN
+        UPDATE totals SET records = records + 1, size = size + NEW.size;
+    END;
+    CREATE TRIGGER totals_after_delete AFTER DELETE ON files BEGIN
+        UPDATE totals SET records = records - 1, size = size - OLD.size;
+    END;
+    CREATE TRIGGER totals_after_update AFTER UPDATE OF size ON files BEGIN
+        UPDATE totals SET size = size + NEW.size - OLD.size;
+    END;
     ```
 
     Paths are stored as BLOBs because Unix paths are raw bytes, not guaranteed
@@ -406,6 +420,17 @@ All four subcommands operate on a single SQLite database file:
     `files_signature` index lets `report` read the records in signature order
     without sorting the table.
 
+    `totals` holds one row: the number of records in `files` and the sum of
+    their sizes. Three triggers on `files` keep that row current. Each runs in
+    the transaction of the change to `files` that fires it, so the row and the
+    records always agree. `info` reads both numbers from this row. `scan`,
+    `report` and `trees` read the number of records from it as the total of
+    their progress displays. Counting the records in `files` would read every
+    record, which takes minutes on a database of tens of millions. The triggers
+    make each write to `files` slower, so a first scan of many small files takes
+    about two fifths longer. On large files, where reading them takes most of
+    the time, the difference is small.
+
 ### Duplicate detection
 
 Two files are duplicates when they have the same size and the same three hashes.
@@ -688,14 +713,18 @@ Processing:
   database state.
 
 `report` runs in two phases, each with its own progress display on stderr (see
-"Progress"): `open` opens the database; `read` counts the records, then reads
-them in signature order, largest size first. That is the order of the
-`files_signature` index read backwards, so SQLite sorts nothing and returns the
-first record at once. The records sharing a signature come together, and so do
-the records of one size, so `report` holds only the duplicate groups of the size
-it is reading, and writes them, ordered by `first` path, once it reaches a
-smaller size. Its memory holds at most the duplicate paths of one size at a
-time, never every record.
+"Progress"):
+
+- `open` opens the database.
+- `read` gets the number of records from `totals`, then reads the records in
+  signature order, largest size first.
+
+Signature order, largest size first, is the `files_signature` index read
+backwards, so SQLite sorts nothing and returns the first record at once. The
+records sharing a signature come together, and so do the records of one size, so
+`report` holds only the duplicate groups of the size it is reading, and writes
+them, ordered by `first` path, once it reaches a smaller size. Its memory holds
+at most the duplicate paths of one size at a time, never every record.
 
 #### Report output format
 
@@ -769,10 +798,13 @@ Processing:
   ascending. Output must be fully deterministic for a given input.
 
 `trees` runs in four phases, each with its own progress display on stderr (see
-"Progress"): `open` opens the database; `read` counts the records, then reads
-them in path order, building the hierarchy and computing the digests as it goes;
-`group` groups the directories by digest and picks the maximal groups; `write`
-writes the groups to stdout.
+"Progress"):
+
+- `open` opens the database.
+- `read` gets the number of records from `totals`, then reads the records in
+  path order, building the hierarchy and computing the digests as it goes.
+- `group` groups the directories by digest and picks the maximal groups.
+- `write` writes the groups to stdout.
 
 #### Trees output format
 
@@ -819,15 +851,19 @@ size of recorded files	150000000000000 (136.4 TiB)
   database is in WAL mode, the newest changes may be in the `-wal` file alone.
 - `schema version` is the database's `PRAGMA user_version`. A database with
   another version than this build's is refused, as by `report`.
-- `records` is the number of records. `size of recorded files` adds up the sizes
-  of all records, in bytes, then in human units.
+- `records` is the number of records. `size of recorded files` is the sum of the
+  sizes of all records, in bytes, then in human units.
 
 `info` runs in two phases, each with its own progress display on stderr (see
-"Progress"): `open` opens the database; `count` reads the schema version and
-counts the records with one query, a single pass over the table, both in one
-read-only transaction, so the numbers agree with each other while a scan is
-committing. `info` writes its lines once the `count` display has ended, so on a
-terminal they come after the displays.
+"Progress"):
+
+- `open` opens the database.
+- `count` reads the schema version and the row of `totals` (see "Database").
+  Both reads run in one read-only transaction, so the numbers agree with each
+  other while a scan is committing.
+
+`info` writes its lines once the `count` display has ended, so on a terminal
+they come after the displays.
 
 ### Progress
 
@@ -843,8 +879,9 @@ progress, its elapsed time and its time remaining.
 A scan has four phases (`[1/4] load`, `[2/4] walk`, `[3/4] hash`,
 `[4/4] update`):
 
-- `load` counts the records first, then reads them against that count. While the
-  count runs, the display says it is counting the records.
+- `load` gets the number of records from `totals` first, then reads the records
+  against that number. Until it has the number, the display says it is counting
+  the records.
 - `walk` has no exact total. Its estimate is the number of records the last scan
   left under the operands, known once `load` is done and shown with a `~`. The
   time remaining is unknown once the walk passes the estimate, and when there
@@ -857,8 +894,9 @@ A scan has four phases (`[1/4] load`, `[2/4] walk`, `[3/4] hash`,
 
 - `open` has no total. While it opens the database, waiting on a busy one
   included, the display says it is opening the database.
-- `read` counts the records first, then reads them against that count. While the
-  count runs, the display says it is counting the records.
+- `read` gets the number of records from `totals` first, then reads the records
+  against that number. Until it has the number, the display says it is counting
+  the records.
 - `group` has the directories `read` found as its total, and `write` the
   duplicate-tree groups.
 
@@ -866,17 +904,16 @@ A scan has four phases (`[1/4] load`, `[2/4] walk`, `[3/4] hash`,
 
 - `open` has no total. While it opens the database, waiting on a busy one
   included, the display says it is opening the database.
-- `read` counts the records first, then reads them against that count, writing
-  the rows to stdout as it goes. While the count runs, the display says it is
-  counting the records.
+- `read` gets the number of records from `totals` first, then reads the records
+  against that number, writing the rows to stdout as it goes. Until it has the
+  number, the display says it is counting the records.
 
 `info` has two phases (`[1/2] open`, `[2/2] count`, see "`info` mode"):
 
 - `open` has no total. While it opens the database, waiting on a busy one
   included, the display says it is opening the database.
-- `count` has no total either, because one query counts everything: the display
-  says it is counting the records until the query returns, and its time
-  remaining is unknown.
+- `count` has no total either, because it reads one row: the display says it is
+  counting the records until it has the row, and its time remaining is unknown.
 
 Each display is one line holding: