PostgreSQL: What the future might have in stock for us. Update for Postgres 9.6. Here, the query prefers the index on (ts, group_id), but I think that's because of the test setup with "clustered" timestamps. Drop the index! It keeps a running total of rows, and whenever the staff id changes, it adds the row to the returned result set. To filter groups, you use the HAVING clause instead of WHERE clause.. D) Using PostgreSQL GROUP BY with multiple columns. Our tips for PostgreSQL query optimization will help you to speed up queries 10-100x for multi-GB databases. In the latest article in my blog: NOT IN vs. NOT EXISTS vs. LEFT JOIN / IS NULL: Oracle, there is a query in which Oracle does not use full table scan but rather joins two indexes to get the column values:

"Loose indexscan" in MySQL, "index skip scan" in Oracle, SQLite, Cockroach (very recently added), and "jump scan" in DB2 are the names used for an operation that finds the the distinct values of the leading columns of a btree index efficiently; rather than scanning all equal values of a key, as soon as a new value is found, restart the search by looking for a larger value. All indexes in PostgreSQL are secondary indexes, meaning that each index is stored separately from the table's main data area (which is called the table's heap in PostgreSQL terminology). In some scenarios my query seems to ignore the index that I've created which is used for joining the two tables data and data_area.When this happens it uses a sequential scan and results in a much slower query. PostgreSQL partial index even allows you to specify the rows of a table that should be indexed. どうも。村上です。 今回がきっと最終回の5回目です。 では、最後となる「Index Only Scan 奮闘記」を。 Index Only Scan の explain 前回でもIndex Only Scanの実行計画をみました。 今回は前回より複雑なIndex Only Scanのexplainをみましょう。 テーブルはこんな感じです。 Thus, no guarantee that optimizer will choose your_wanted_index, it can be so that postgresql engine will just perform a sequence / primary key scan instead.

Note that it is possible to examine the number of index scans (including index-only scans and bitmap index scans) by examining pg_stat_user_indexes.idx_scan.

The following example uses multiple columns in the GROUP BY clause: どうも。村上です。 今回がきっと最終回の5回目です。 では、最後となる「Index Only Scan 奮闘記」を。 Index Only Scan の explain 前回でもIndex Only Scanの実行計画をみました。 今回は前回より複雑なIndex Only Scanのexplainをみましょう。 テーブルはこんな感じです。 As of version 10.x PostgreSQL always has to join first and aggregate later. An index, however, will hardly be useful (per se), because the query has to read the whole table anyway - the exception being index-only scans in Postgres 9.2+ and favorable conditions, see below. The first thing I would try is replacing the IN with an EXISTS:. If your covering index isn't being used, you're essentially paying for the overhead of maintaining it during writes with no benefit in return.

I have a table with a multicolumn index, and I have doubts about the proper sorting of the indexes to get the maximum performance on the queries. When the conditions for a Loose Index Scan are not met, it still may be possible to avoid creation of temporary tables for GROUP BY queries. Summary If you have a large table, Oracle can prefer deriving the fields from the indexes rather than from the table, even there is no single index that covers all values.

Index.