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Update memory.md

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Yarik Briukhovetskyi 2024-03-15 15:50:17 +01:00 committed by GitHub
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docs/en/engines/table-engines/special/memory.md
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@ -22,8 +22,7 @@ Normally, using this table engine is not justified. However, it can be used for
The Memory engine is used by the system for temporary tables with external query data (see the section “External data for processing a query”), and for implementing `GLOBAL IN` (see the section “IN operators”).
Upper and lower bounds can be specified to limit Memory engine table size, effectively allowing it to act as a circular
buffer (see [Engine Parameters](#engine-parameters)).
Upper and lower bounds can be specified to limit Memory engine table size, effectively allowing it to act as a circular buffer (see [Engine Parameters](#engine-parameters)).
## Engine Parameters {#engine-parameters}
@ -40,9 +39,6 @@ buffer (see [Engine Parameters](#engine-parameters)).
## Usage {#usage}
``` sql
CREATE TABLE memory (i UInt32) ENGINE = Memory;
```
**Initialize settings**
``` sql
@ -56,16 +52,42 @@ CREATE TABLE memory (i UInt32) ENGINE = Memory SETTINGS min_rows_to_keep = 100,
CREATE TABLE memory (i UInt32) ENGINE = Memory SETTINGS min_bytes_to_keep = 4096, max_bytes_to_keep = 16384;
/* 1. testing oldest block doesn't get deleted due to min-threshold - 3000 rows */
INSERT INTO memory SELECT * FROM numbers(0, 1600);
INSERT INTO memory SELECT * FROM numbers(0, 1600); -- 8'192 bytes
/* 2. adding block that doesn't get deleted */
INSERT INTO memory SELECT * FROM numbers(1000, 100);
INSERT INTO memory SELECT * FROM numbers(1000, 100); -- 1'024 bytes
/* 3. testing oldest block gets deleted - 9216 bytes - 1100 */
INSERT INTO memory SELECT * FROM numbers(9000, 1000);
INSERT INTO memory SELECT * FROM numbers(9000, 1000); -- 8'192 bytes
/* 4. checking a very large block overrides all */
INSERT INTO memory SELECT * FROM numbers(9000, 10000);
INSERT INTO memory SELECT * FROM numbers(9000, 10000); -- 65'536 bytes
SELECT total_bytes, total_rows FROM system.tables WHERE name = 'memory' and database = currentDatabase();
```
``` text
┌─total_bytes─┬─total_rows─┐
│ 65536 │ 10000 │
└─────────────┴────────────┘
```
also, for rows:
``` sql
CREATE TABLE memory (i UInt32) ENGINE = Memory SETTINGS min_rows_to_keep = 4000, max_rows_to_keep = 10000;
/* 1. testing oldest block doesn't get deleted due to min-threshold - 3000 rows */
INSERT INTO memory SELECT * FROM numbers(0, 1600); -- 1'600 rows
/* 2. adding block that doesn't get deleted */
INSERT INTO memory SELECT * FROM numbers(1000, 100); -- 100 rows
/* 3. testing oldest block gets deleted - 9216 bytes - 1100 */
INSERT INTO memory SELECT * FROM numbers(9000, 1000); -- 1'000 rows
/* 4. checking a very large block overrides all */
INSERT INTO memory SELECT * FROM numbers(9000, 10000); -- 10'000 rows
SELECT total_bytes, total_rows FROM system.tables WHERE name = 'memory' and database = currentDatabase();
```