In MySQL, when I create a composite primary key, say with columns X, Y, Z, then all three columns become indexes automatically. Does the same happen for Postgres?
In MySQL, when I create a composite primary key, say with columns X, Y,
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In MySQL, when I create a composite primary key, say with columns X, Y, Z, then all three columns become indexes automatically. Does the same happen for Postgres?
If you create a composite primary key, on
(x, y, z), PostgreSQL implements this with the help of oneUNIQUEmulti-column btree index on(x, y, z). In addition, all three columns areNOT NULL(implicitly), which is the main difference between aPRIMARY KEYand aUNIQUE INDEX.Besides obvious restrictions on your data, the multi-column index also has a somewhat different effect on the performance of queries than three individual indexes on
x,yandz.Related discussion on dba.SE:
With examples, benchmarks, discussion and outlook on the new feature of index-only scans in Postgres 9.2.
In particular, a primary key on
(x, y, z)will speed up queries with conditions onx,(x,y)or(x,y,z)optimally. It will also help with queries ony,z,(y,z)or(x,z)but to a far lesser extent.If you need to speed up queries on the latter combinations, you may want to change the order of column in your PK constraint and/or create one or more additional indexes. See: