Labs

Estimate versus actual on a correlated filter

A PostgreSQL table where postcode implies city: the row estimate against the actual count, with per-column statistics, dependencies and an MCV list.

Join elimination with and without an enforced constraint

PostgreSQL removes a LEFT JOIN to a unique key it does not need; an inner join stays. Then an unenforced key and an orphan row change the answer.

Find the index-versus-scan crossover on your machine

Force index and sequential scans across selectivities from 0.01% to 50%, on clustered and shuffled data, and compare with what the planner picks.

A partition with no statistics

Load a new day into a partitioned PostgreSQL table and compare the plan before and after ANALYZE on that partition alone.

Cross the dynamic-programming cliff

Plan joins of 8 to 16 tables in a chain and a star with PostgreSQL's exhaustive search and with GEQO; count distinct plans.

Four pairs of equal queries

NOT IN against NOT EXISTS with a NULL, a function on a partition key against a range, and a CTE materialized or not, in PostgreSQL.

Min/max skipping on sorted versus unsorted data

10 million store_ids in Parquet, random and sorted: how many of 100 row groups a filter must read, and what one outlier does.

Pushdown and projection: count the bytes

DuckDB on Parquet: a filter on sorted and random files, all columns against one, and a filter wrapped in an expression.

Top-N heap against full sort

The top 100 of 20 million numbers with a sort and with a heap; then a PostgreSQL LIMIT that makes the plan worse.

Bloom filter size and false-positive rate

Bloom filters for a million keys at 5, 10 and 15 bits per key: false-positive rate and size.

Late materialization crossover

Filter a 100-million-element array with a boolean mask and with a position list, across selectivities, and find where gathering loses.

Parameter sniffing in PostgreSQL

A prepared statement on a 95%/0.002% skewed column: custom plans, generic plans, and when PostgreSQL switches between them.