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May 13, 2026
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Tweak markdown formatting, fix a few errors #249

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53 changes: 26 additions & 27 deletions doc/pg_clickhouse.md
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Original file line number Diff line number Diff line change
Expand Up @@ -188,7 +188,7 @@ CREATE USER MAPPING FOR CURRENT_USER SERVER taxi_srv
OPTIONS (user 'demo');
```

The The supported options are:
The supported options are:

* `user`: The name of the ClickHouse user. Defaults to "default".
* `password`: The password of the ClickHouse user.
Expand Down Expand Up @@ -691,7 +691,7 @@ pg_clickhouse parameters before executing queries that depend on them.

### SET

Use [SET] to set the the pg_clickhouse custom configuration parameters.
Use [SET] to set the pg_clickhouse custom configuration parameters.

#### `pg_clickhouse.session_settings`

Expand Down Expand Up @@ -1288,15 +1288,12 @@ Ranking functions (`row_number`, `rank`, `dense_rank`, `ntile`, `cume_dist`,
`percent_rank`) omit their frame clause during pushdown because ClickHouse
rejects frame specifications on these functions.

[window functions]: https://www.postgresql.org/docs/current/functions-window.html
"PostgreSQL Docs: Window Functions"

## Compatibility Notes

### Regular Expressions

While pg_clickhouse pushes down regular expressions to ClickHouse equivalents
when [pg_clickhouse.pushdown_regex](pg_clickhousepushdown_regex) is true (the
when [pg_clickhouse.pushdown_regex](#pg_clickhousepushdown_regex) is true (the
default), and makes an effort to ensure a basic level of compatibility, be
aware of the differences between the two and how pg_clickhouse handles them.

Expand Down Expand Up @@ -1328,14 +1325,14 @@ aware of the differences between the two and how pg_clickhouse handles them.
* The only flags both support, and therefore can be used when evaluated by
ClickHouse, are:

RE2 supports only these flags; don't use any other [Postgres flags]

* `i`: case-insensitive
* `m`: multi-line mode:
* `s`: let `.` match `\n`
* `p`: partial newline-sensitive matching (treated the same as `s`)
* `t`: tight syntax (the default, removed by pg_clickhouse)

RE2 supports only these flags; don't use any other [Postgres flags]

* Any other flags passed to regular expression functions will cause the
function not to be pushed down.

Expand All @@ -1349,12 +1346,12 @@ aware of the differences between the two and how pg_clickhouse handles them.
ClickHouse.

To avoid all ambiguity, consider setting
[pg_clickhouse.pushdown_regex](pg_clickhousepushdown_regex) to prevent
[pg_clickhouse.pushdown_regex](#pg_clickhousepushdown_regex) to prevent
Postgres regular expression from pushing down to ClickHouse, and using the
[re2 extension], for which pg_clickhouse supports [direct pushdown](#re2)
of ClickHouse-compatible [RE2] regular expressions.
[re2 extension], for which pg_clickhouse supports [direct pushdown](#re2) of
ClickHouse-compatible [RE2] regular expressions.

### to_char()
### `to_char()`

PostgreSQL [`to_char()`] for `timestamp` and `timestamp with time zone`
pushes down to ClickHouse [formatDateTime] only when the format argument
Expand All @@ -1370,21 +1367,21 @@ formats date-time values.

#### Translated keywords

| PostgreSQL | ClickHouse | Meaning |
| ---------- | ---------- | ------- |
| `YYYY`, `yyyy` | `%Y` | 4-digit year |
| `YY`, `yy` | `%y` | 2-digit year |
| `MM`, `mm` | `%m` | zero-padded month (01–12) |
| `DD`, `dd` | `%d` | zero-padded day of month (01–31) |
| `DDD`, `ddd` | `%j` | zero-padded day of year (001–366) |
| `HH24`, `hh24` | `%H` | zero-padded 24-hour (00–23) |
| `HH`, `hh`, `HH12`, `hh12` | `%I` | zero-padded 12-hour (01–12) |
| `MI`, `mi` | `%i` | zero-padded minute (00–59) |
| `SS`, `ss` | `%S` | zero-padded second (00–59) |
| `Q`, `q` | `%Q` | quarter (1–4) |
| `Mon` | `%b` | abbreviated month name, e.g., `Oct` |
| `Dy` | `%a` | abbreviated weekday name, e.g., `Mon` |
| `AM`, `PM` | `%p` | meridiem indicator, always uppercase |
| PostgreSQL | ClickHouse | Meaning |
| -------------------------- | ---------- | ------------------------------------- |
| `YYYY`, `yyyy` | `%Y` | 4-digit year |
| `YY`, `yy` | `%y` | 2-digit year |
| `MM`, `mm` | `%m` | zero-padded month (01–12) |
| `DD`, `dd` | `%d` | zero-padded day of month (01–31) |
| `DDD`, `ddd` | `%j` | zero-padded day of year (001–366) |
| `HH24`, `hh24` | `%H` | zero-padded 24-hour (00–23) |
| `HH`, `hh`, `HH12`, `hh12` | `%I` | zero-padded 12-hour (01–12) |
| `MI`, `mi` | `%i` | zero-padded minute (00–59) |
| `SS`, `ss` | `%S` | zero-padded second (00–59) |
| `Q`, `q` | `%Q` | quarter (1–4) |
| `Mon` | `%b` | abbreviated month name, e.g., `Oct` |
| `Dy` | `%a` | abbreviated weekday name, e.g., `Mon` |
| `AM`, `PM` | `%p` | meridiem indicator, always uppercase |

#### Quoted text and literals

Expand Down Expand Up @@ -1492,6 +1489,8 @@ Copyright (c) 2025-2026, ClickHouse.
"ClickHouse Docs: String"
[TEXT]: https://www.postgresql.org/docs/current/datatype-character.html
"PostgreSQL Docs: Character Types"
[window functions]: https://www.postgresql.org/docs/current/functions-window.html
"PostgreSQL Docs: Window Functions"
[POSIX Regular Expressions]: https://www.postgresql.org/docs/18/functions-matching.html#FUNCTIONS-POSIX-REGEXP
"PostgreSQL Docs: POSIX Regular Expressions"
[Postgres flags]: https://www.postgresql.org/docs/18/functions-matching.html#POSIX-EMBEDDED-OPTIONS-TABLE
Expand Down
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