How a report is made

pgloupe reads inputs and builds one report in stages. Each stage hands the next only what it needs.

logs ──► decoder ──► assembler ──► classifier ──► accumulators ──► report
                                                         ▲
snapshots ──► delta (reset and eviction aware) ───────────┘

1. Decoder. Reads stderr, csvlog or jsonlog, plain or compressed. For stderr it infers the log_line_prefix from a sample and prints the one it chose; if no candidate matches nearly every line it says so and stops. Servers that log in Portuguese (Brazil), Spanish, French or German are read through a translation table (experimental).
2. Assembler. Joins continuation lines to their entry, per backend process, and attaches DETAIL, HINT, CONTEXT and STATEMENT lines to the entry they belong to.
3. Classifier. Turns each entry into an event: a statement with or without a duration, a lock wait, a temp file, a checkpoint, an autovacuum run, a connection, an error, and so on.
4. Correlation. Connects a duration to the statement it belongs to when the server logged them apart (cursors, prepared statements, parse/bind/execute), and names the statement that held a lock another one waited for.
5. Accumulators. Count and keep, per statement fingerprint, calls, time, percentiles, errors, lock waits and temp files. A fingerprint comes from the statement’s parse tree (libpg_query), so WHERE id = 1 and WHERE id = 2 are one statement.
6. Join. If you gave snapshots, pg_stat_statements counts for the window are matched to the log’s statements by fingerprint. pgloupe notices resets, evictions and windows that do not overlap, and says so instead of showing a wrong number. pg_stat_database from the same two snapshots gives the server’s load over the window.
7. Findings and advice. Rules turn the numbers into What to fix first and into Settings advice, each with the evidence behind it.

With a store (logs ingest), the accumulators’ per-hour results are kept in SQLite and a report for any window is built from them without decoding the logs again.

What it does not do

It reads files you give it or fetch with its own commands; it is not a monitoring system and it never changes your server. The only statements it runs are read-only queries against pg_stat_statements, pg_stat_database, pg_settings and a few catalog views.