count(distinct ...) always sorts, rather than using a hash, to do its work. I don't think that there is any fundamental reason that it could not be changed to allow it to use hashing, it just hasn't been done ye
PostgreSQL CASEs can have multiple conditions. There are a handful of entries in the table where the name of country and capital is the same. You can add this to your previously constructed CASE - SELECT name, continent, indep_year, CASE WHEN (indep_year < 1900) AND (countries.name = co...
Another option is to use range partitioning with multiple columns in the partition key. Either of these can easily lead to excessive numbers of partitions, so restraint is advisable. It is important to consider the overhead of partitioning during query planning and execution. The query planner is...
*/ resultSlot = ExecProject(projInfo, &isDone); if (isDone != ExprEndResult) { node->ps.ps_TupFromTlist = (isDone == ExprMultipleResult); return resultSlot; } } else { /* * Here, we aren't projecting, so just return scan tuple. */ return slot; } } else InstrCountFiltered1(n...
*/ StaticAssertDecl(THRESHOLD_SUBTRANS_CLOG_OPT <= PGPROC_MAX_CACHED_SUBXIDS, "group clog threshold less than PGPROC cached subxids"); /* * When there is contention on XactSLRULock, we try to group multiple * updates; a single leader process will perform transaction status * updates ...
If any of the upper 25/17 bits are not zero, the code point is broken into multiple 1-byte (UTF-8) or 2-byte (UTF-16) sequences based on someclever rules, One of the very nice features of Unicode is that any code point where the upper 25 bits are zero is exactly the same as...
Composite Types –show you how to define a composite type that consists of multiple fields. Section 15. Conditional Expressions & Operators CASE –show you how to form conditional queries with CASE expression. COALESCE –return the first non-null argument. You can use it to substitute NULL by ...
Additionally, PostgreSQL supports multiple languages across triggers, foreign key attributes, joins, and stored procedures. PostgreSQL allows for the most common data types, including SQL 2008, and it supports Unicode, international character sets, and multibyte character encodings. PostgreSQL is an open...
when 'ExclusiveLock' then return 7; when 'AccessExclusiveLock' then return 8; else return 0; end case; end; $$ language plpgsql strict; 修改查询语句,按锁级别排序: with t_wait as (select a.mode,a.locktype,a.database,a.relation,a.page,a.tuple,a.classid,a.objid,a.objsubid, a....
(To reduce context switching, when multiple transactions commit nearly simultaneously, we have one backend take ProcArrayLock and clear the XIDs of multiple processes at once.) ProcArrayEndTransaction also holds the lock while advancing the shared ...